Image Forming Apparatus

ABSTRACT

An image forming apparatus includes a switchback roller which switches between a normal rotation direction and a reverse rotation direction, first and second gear train for rotating the switchback roller in the normal and reverse rotation directions, respectively, a switching unit including a pendulum gear which is movable among a first engagement position where the pendulum gear is engaged with the first gear train, a second engagement position where the pendulum gear is engaged with the second gear train, and a disengagement position where the pendulum gear is not engaged with the first gear train and the second gear train. The switching unit is switchable among a first mode where the pendulum gear is held at the first engagement position, a second mode where the pendulum gear is held at the second engagement position, and a third mode where the pendulum gear is held at the disengagement position.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority from Japanese Patent Application No.2013-075320, filed on Mar. 29, 2013, the entire subject matter of whichis incorporated herein by reference.

TECHNICAL FIELD

Aspects of the present invention relate to an image forming apparatusemploying an electro-photographic system.

BACKGROUND

There have been known a printer which consecutively prints both sides ofa plurality of sheets.

As such printer, there has been proposed a printer in which after animage is formed on one side of a sheet, a discharge roller is rotated ina reverse direction such that the sheet is re-conveyed into a main bodycasing (switchback conveyance), and an image is formed on the other sideof the sheet (for example, JP-A-2011-048328).

Further, in the printer disclosed in JP-A-2011-048328, as a drivingsource for various rollers, in addition to a motor for rotatingphotosensitive drums and developing rollers in one direction and a motorfor rotating rollers for conveying sheets toward the image forming unitin one direction, there would be necessary to provide a motor forrotating the discharge roller which switches between a normal rotationand a reverse rotation. Therefore, cost may increase and noise may begenerated from the motor sounds.

SUMMARY

Accordingly, an aspect of the present invention provides an imageforming apparatus capable of switching a conveyance direction of arecording medium by a simple configuration so as to form images on oneside and the other side of the recording medium while reducing cost andnoise.

According to an illustrative embodiment of the present invention, thereis provided an image forming apparatus comprising: a driving sourceconfigured to generate one-direction rotational driving force; aswitchback roller configured to be switched between a normal rotationdirection and a reverse rotation direction for switching a conveyancedirection of a recording medium having an image formed thereon by animage forming unit; a first gear train configured to transmit theone-direction rotational driving force of the driving source to theswitchback roller such that a rotation direction of the switchbackroller becomes the normal rotation direction; a second gear trainconfigured to transmit the one-direction rotational driving force of thedriving source to the switchback roller such that the rotation directionof the switchback roller becomes the reverse rotation direction; and aswitching unit including a pendulum gear configured to engage a geartrain for transmitting the one-direction rotational driving force of thedriving source with any of the first gear train and the second geartrain. The pendulum gear is configured to be movable among: a firstengagement position where the pendulum gear is engaged with the firstgear train, a second engagement position where the pendulum gear isengaged with the second gear train, and a disengagement position wherethe pendulum gear is not engaged with any of the first gear train andthe second gear train. The switching unit is configured to be switchableamong: a first mode in which the pendulum gear is held at the firstengagement position such that the one-direction rotational driving forceof the driving source is transmitted to the first gear train, a secondmode in which the pendulum gear is held at the second engagementposition such that the one-direction rotational driving force of thedriving source is transmitted to the second gear train, and a third modein which the pendulum gear is held at the disengagement position suchthat the one-direction rotational driving force of the driving source isnot transmitted to any of the first gear train and the second geartrain.

According to this configuration, if the pendulum gear is held at thefirst engagement position such that the one-direction rotational drivingforce of the driving source is transmitted to the first gear train, theswitching unit becomes the first mode in which the rotation direction ofthe switchback roller becomes the normal rotation direction, and if thependulum gear is held at the second engagement position such that theone-direction rotational driving force of the driving source istransmitted to the second gear train, the switching unit becomes thesecond mode in which the rotation direction of the switchback rollerbecomes the reverse rotation direction, and if the pendulum gear is heldat the disengagement position such that the one-direction rotationaldriving force of the driving source is not transmitted to any of thefirst gear train and the second gear train, the switching unit becomesthe third mode in which the switchback roller does not rotate.

Therefore, by moving the pendulum gear to the first engagement position,the second engagement position, and the disengagement position in astate where the one-direction rotational driving force of the drivingsource is generated, and holding the pendulum gear at each engagementposition by the switching unit, it is possible to perform switching tothe first mode, the second mode, and the third mode.

As a result, it is not necessary to switch the rotation direction of therotational driving force of the driving source for switching therotation direction of the switchback roller, and thus it is possible touse one driving source not only as a driving source for generatingrotational driving force for rotating rotary bodies which are in theimage forming apparatus and rotate in one direction, but also as adriving source for generating rotational driving force to be transmittedto the switchback roller.

Therefore, it is possible to prevent the number of driving sources inthe image processing apparatus, and it is possible to switch therotation direction of the switchback roller between the normal rotationdirection and the reverse rotation direction, thereby forming images onone side and the other side of each recording medium while reducing thecost and noise.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other aspects of the present invention will become moreapparent and more readily appreciated from the following description ofillustrative embodiments of the present invention taken in conjunctionwith the attached drawings, in which:

FIG. 1 is a center cross-sectional view showing a printer which is anexample of an image forming apparatus according to an illustrativeembodiment of the present invention;

FIGS. 2A and 2B are block diagrams showing a drive transmission systemof the printer shown in FIG. 1, wherein FIG. 2A shows a block diagram ofa main motor, and FIG. 2B shows a block diagram of a process motor;

FIG. 3 is a rear view showing a driving-force transmission mechanismwhich is configured inside the printer shown in FIG. 1;

FIG. 4 is a side view showing the driving-force transmission mechanismof FIG. 3 in a first mode;

FIG. 5 is a cross-sectional view of the driving-force transmissionmechanism of FIG. 3 in the first mode as taken along a line A-A;

FIG. 6 is a cross-sectional view of the driving-force transmissionmechanism of FIG. 3 in the first mode as taken along a line B-B;

FIGS. 7A and 7B are views showing a partially toothed gear of a sectorgear, a lever, and a drive gear shown in FIG. 3, in the first mode,wherein FIG. 7A is a cross-sectional view taken along a line C-C shownin FIG. 3, and FIG. 7B is a perspective view as seen from the upper rearside;

FIGS. 8A to 8D are views showing the sector gear of FIG. 4, wherein FIG.8A is a right side view, and FIG. 8B is a rear view, and FIG. 8C is aleft side view, and FIG. 8D is a perspective view as seen from the upperfront side, and wherein for the sake of convenience, directions arebased on the posture of the sector gear in a normal rotation mode;

FIG. 9 is a side view showing the driving-force transmission mechanismof FIG. 3 in a second mode;

FIG. 10 is a cross-sectional view showing the driving-force transmissionmechanism of FIG. 3 in the second mode as taken along the line A-A;

FIG. 11 is a cross-sectional view showing the driving-force transmissionmechanism of FIG. 3 in the second mode as taken along the line B-B;

FIGS. 12A and 12B are views showing the partially toothed gear, thelever, and the drive gear of the sector gear of FIG. 3 in the secondmode, wherein FIG. 12A is a cross-sectional view taken along a line D-Dshown in FIG. 3, and FIG. 12B is a perspective view as seen from theupper rear side;

FIG. 13 is a side view showing the driving-force transmission mechanismof FIG. 3 in a third mode;

FIG. 14 is a cross-sectional view showing the driving-force transmissionmechanism of FIG. 3 in the third mode as taken along the line A-A;

FIG. 15 is a cross-sectional view showing the driving-force transmissionmechanism of FIG. 3 in the third mode as taken along the line B-B;

FIGS. 16A and 16B are views showing the partially toothed gear of thesector gear, the lever, and the drive gear shown in FIG. 3, in the thirdmode, wherein FIG. 16A is a cross-sectional view taken along the lineC-C shown in FIG. 3, and FIG. 16B is a perspective view as seen from theupper rear side;

FIG. 17 is a block diagram showing a flow of control in the printershown in FIG. 1;

FIG. 18 is a timing chart for explaining the operation of each unitimmediately after power-on;

FIG. 19 is a timing chart for explaining a double-sided image formingprocess;

FIGS. 20A to 20D are explanatory views for explaining sheet conveyancein the double-sided image forming process, wherein FIG. 20A correspondsto a timing A of FIG. 19, and FIG. 20B corresponds to a timing B of FIG.19, and FIG. 20C corresponds to a timing C of FIG. 19, and FIG. 20Dcorresponds to a timing D of FIG. 19;

FIGS. 21E to 21H are explanatory views for explaining the sheetconveyance in the double-sided image forming process subsequent to FIGS.20A to 20D, wherein FIG. 21E corresponds to a timing E of FIG. 19, andFIG. 21F corresponds to a timing F of FIG. 19, and FIG. 21G correspondsto a timing G of FIG. 19, and FIG. 21H corresponds to a timing H of FIG.19; and

FIGS. 22I to 22L are explanatory views for explaining the sheetconveyance in the double-sided image forming process subsequent to FIGS.21E to 21H, wherein FIG. 221 corresponds to a timing I of FIG. 19, andFIG. 22J corresponds to a timing J of FIG. 19, and FIG. 22K correspondsto a timing K of FIG. 19, and FIG. 22L corresponds to a timing L of FIG.19.

DETAILED DESCRIPTION 1. Overall Configuration of Printer

As shown in FIG. 1, a printer 1 (an example of an image formingapparatus) is a direct tandem type color laser printer. The printer 1includes, inside a main body casing 2, a sheet feeding unit 3 forfeeding a sheet P (an example of a recording medium), an image formingunit 4 for forming an image on the fed sheet P, a sheet discharge unit 5for discharging the sheet P having the image formed thereon, and areverse conveyance unit 6 for re-conveying the sheet P having the imageformed thereon into the image forming unit 4.

In the following description, in case of referring to directions of theprinter 1, the upper side and the lower side of the printer are based ona state where the printer 1 is installed horizontally. That is, theupper side of the sheet of FIG. 1 is the upper side of the printer, andthe lower side of the sheet of FIG. 1 is the lower side of the printer.Further, the right side of the sheet of FIG. 1 is the front side of theprinter, and the left side of the sheet of FIG. 1 is the rear side ofthe printer. Also, the left and right of the printer 1 are based ondirections as the printer 1 is viewed from the front side.

That is, a direction toward a viewer of FIG. 1 is the left side of theprinter, and a direction away from the viewer of FIG. 1 is the rightside of the printer.

(1) Main Body Casing

The main body casing 2 is formed in a box shape having a substantiallyrectangular shape as seen in a side view, and accommodates the sheetfeeding unit 3, the image forming unit 4, the sheet discharge unit 5,and the reverse conveyance unit 6. The main body casing 2 has a frontwall having a main body opening 9, and a front cover 10. The front cover10 is configured to be able to swing around its lower end portion, so asto open or close the main body opening 9.

(2) Sheet Feeding Unit

The sheet feeding unit 3 is configured to convey sheets P toward theimage forming unit 4. The sheet feeding unit 3 includes a sheet feedingtray 12, a pickup roller 13, a sheet feeding roller 14, a sheet feedingpad 15, a conveying roller 16, and a registration roller 17.

The sheet feeding tray 12 accommodates sheets P and is removably set ata lower portion of the inside of the main body casing 2. The sheets P onthe sheet feeding tray 12 are sent into a space between the sheetfeeding roller 14 and the sheet feeding pad 15 by rotation of the pickuproller 13, and are separated one by one by rotation of the sheet feedingroller 14.

The conveying roller 16 is positioned in a substantially U-shapedconveyance path extending from the sheet feeding roller 14 to the imageforming unit 4, and conveys a sheet P having been conveyed from thesheet feeding roller 14, toward the registration roller 17.

The registration roller 17 is positioned on the downstream side from theconveying roller 16 in the conveyance direction of the sheet P and onthe upstream side from the image forming unit 4 in the conveyancedirection of the sheet P. The registration roller 17 contacts the sheetP having been conveyed from the conveying roller 16, thereby correctingskew of the sheet P. Thereafter, the registration roller 17 is rotatedin a normal rotation direction, so that the sheet P is conveyed at apredetermined timing toward between photosensitive drums 28 (to bedescribed below) and a conveyor belt 39 (to be described below) providedin the image forming unit 4.

(3) Image Forming Unit

The image forming unit 4 includes a scanner unit 20, a drawer unit 21, atransfer unit 22, and a fixing unit 23.

(3-1) Scanner Unit

The scanner unit 20 is disposed at an upper portion of the main bodycasing 2. The scanner unit 20 emits laser beams toward a plurality ofphotosensitive drums 28 (to be described below), that is, fourphotosensitive drums 28, respectively, based on image data, therebyexposing the photosensitive drums 28 (to be described below).

(3-2) Drawer Unit

The drawer unit 21 is disposed below the scanner unit 20 substantiallyat the center of the main body casing 2 in a vertical direction. Thedrawer unit 21 is configured to be slidable in a front-rear direction,and be able to be pulled out from the main body casing 2 through themain body opening 9. The drawer unit 21 includes one process unit 27,and a plurality of developing cartridges 30, that is, four developingcartridges 30.

The process unit 27 includes a plurality of photosensitive drums 28,that is, four photosensitive drums 28, and a plurality of scorotron typechargers 29, that is, four scorotron type chargers 29, corresponding torespective colors.

The plurality of photosensitive drums 28 are disposed in parallel atintervals in the front-rear direction. Specifically, from the front sidetoward rear side of the process unit 27, a black photosensitive drum28K, a yellow photosensitive drum 28Y, a magenta photosensitive drum28M, and a cyan photosensitive drum 28C are sequentially arranged.

The photosensitive drums 28 are formed in a substantially cylindricalshape long in a left-right direction, and are rotatably supported at alower end portion of the process unit 27 such that the photosensitivedrums 28 are exposed from below.

The plurality of scorotron type chargers 29 are provided correspondinglyto the plurality of photosensitive drums 28, respectively. The scorotrontype chargers 29 are positioned on the upper rear sides of correspondingphotosensitive drums 28 with gaps from the photosensitive drums 28,respectively.

The plurality of developing cartridges 30 are provided correspondinglyto the plurality of photosensitive drums 28, respectively. Thedeveloping cartridges 30 are removably installed into the process unit27 so as to be positioned above corresponding photosensitive drums 28,respectively. Each developing cartridge 30 includes a developing roller31, a supply roller 32, and a layer-thickness regulating blade 33.

The developing rollers 31 of the plurality of developing cartridges 30correspond to the colors of the plurality of photosensitive drums 28,respectively. A black developing roller 31K, a yellow developing roller31Y, a magenta developing roller 31M, and a cyan developing roller 31Care sequentially arranged from the front side toward the rear side.

The developing rollers 31 are formed in a substantially columnar shapelong in the left-right direction, and are in contact with the upperfront sides of the photosensitive drums 28.

The supply rollers 32 are formed in a substantially columnar shape longin the left-right direction, and are in contact with the upper frontsides of the developing rollers 31.

The layer-thickness regulating blades 33 are in contact with the uppersides of the developing rollers 31.

The plurality of developing cartridges 30 accommodate tonercorresponding to the respective colors inside their upper spaces,respectively.

The toner in the developing cartridges 30 is fed to the supply rollers32, and is supplied to the developing rollers 31, and is positively andfriction-electrically charged between the supply rollers 32 and thedeveloping rollers 31.

The thickness of the toner having been supplied to the developingrollers 31 is regulated by the layer-thickness regulating blades 33according to rotation of the developing rollers 31, so that the toner iscarried on the surfaces of the developing rollers 31 as thin layershaving a constant thickness.

Incidentally, the surfaces of the photosensitive drums 28 are uniformlyand positively charged by the scorotron type chargers 29 according torotation of the photosensitive drums 28, and then are exposed byhigh-speed scanning with laser beams from the scanner unit 20. As aresult, electrostatic latent images corresponding to an image to beformed on the sheet P are formed on the surfaces of the photosensitivedrums 28, respectively.

When the photosensitive drums 28 further rotate, the toner having beencarried on the surfaces of the developing rollers 31 and having beenpositively charged is supplied to the electrostatic latent images formedon the surfaces of the photosensitive drums 28. As a result, tonerimages are carried on the surfaces of the photosensitive drums 28 byreversal development.

(3-3) Transfer Unit

The transfer unit 22 is disposed along the front-rear direction at aninternal portion of the main body casing 2 which is above the sheetfeeding unit 3 and below the drawer unit 21. This transfer unit 22includes a driving roller 37 and a driven roller 38 which are positionedwith an interval in the front-rear direction, the conveyor belt 39 whichis wound around the driving roller 37 and the driven roller 38, aplurality of transfer rollers 41, that is, four transfer rollers 41which are positioned to sandwich the upper portion of the conveyor belt39 with the plurality of photosensitive drums 28, respectively, and abelt cleaning roller 42 which faces the lower portion of the conveyorbelt 39.

The sheet P having been fed from the sheet feeding unit 3 is conveyedfrom the front side toward the rear side by the conveyor belt 39, so asto pass transfer positions sequentially where the photosensitive drums28 and the transfer rollers 41 face each other. Further, the tonerimages of the respective colors having been carried on thephotosensitive drums 28 are sequentially transferred onto the sheet Pduring the conveyance of the sheet P.

The residual toner on the conveyor belt 39 is cleaned by the beltcleaning roller 42.

(3-4) Fixing Unit

The fixing unit 23 is positioned at the rear of the transfer unit 22,and includes a heating roller 43, and a pressing roller 44 which abutson the lower rear side of the heating roller 43. In the transfer unit22, while the sheet P passes between the heating roller 43 and thepressing roller 44, the color image having been transferred on the sheetP is heated and pressed, thereby being thermally fixed on the sheet P.

(4) Sheet Discharge Unit

The sheet discharge unit 5 is configured to convey a sheet P having animage formed in the image forming unit 4 toward the outside of the mainbody casing 2, or to convey a sheet P having been switched by aswitchback roller 50 toward the reverse conveyance unit 6. The sheetdischarge unit 5 includes a flapper 47, an intermediate sheet dischargeroller 48, the switchback roller 50, a discharge opening 49, and a sheetdischarge tray 51.

The intermediate sheet discharge roller 48 is supported on the main bodycasing 2 at a rear portion substantially at the center of the main bodycasing 2 in the vertical direction such that the rotation direction ofthe intermediate sheet discharge roller can be switched between a normalrotation direction and a reverse rotation direction.

The switchback roller 50 is supported on the main body casing 2 at anupper rear portion of the main body casing 2 such that the rotationdirection of the switchback roller 50 can be switched between a normalrotation direction and a reverse rotation direction. Specifically, theswitchback roller 50 is configured such that the rotation direction ofthe switchback roller 50 can be switched between the normal rotationdirection for conveying a sheet P toward the sheet discharge tray 51through the discharge opening 49, and the reverse rotation direction fordrawing a sheet P having been conveyed toward the sheet discharge tray51 into the main body casing 2, by a switching unit 83 (to be describedbelow).

The discharge opening 49 is an opening for discharging a sheet P havingan image formed in the image forming unit 4 and having been conveyed bythe switchback roller 50 rotating in the normal rotation direction tothe outside of the main body casing 2.

The sheet discharge tray 51 is formed at an upper portion of the mainbody casing 2, substantially in a letter “V” shape having an open upperside as seen in a side view.

The flapper 47 is configured on the downstream side of the fixing unit23 in the conveyance direction of the sheet P such that the flapper 47can be switched between a sheet discharge position and a re-conveyanceposition. The flapper 47 positioned at the sheet discharge positionguides a sheet P having been thermally fixed in the fixing unit 23toward the intermediate sheet discharge roller 48. The flapper 47positioned at the re-conveyance position guides a sheet P having beenreversed by the switchback roller 50 toward the reverse conveyance unit6 formed below the sheet discharge unit 5.

A path in which a sheet P having been fed to the sheet feeding roller 14is conveyed to the conveying roller 16, and passes through the imageforming unit 4, and is conveyed to the switchback roller 50 of the sheetdischarge unit 5 is referred to as a primary conveyance path 52.

(5) Reverse Conveyance Unit

The reverse conveyance unit 6 is configured to convey a sheet P from therear side to front side of the main body casing 2. The reverseconveyance unit 6 is formed to extend from the lower side of the flapper47 and passes under the sheet feeding unit 3 and join the upstream sidefrom the image forming unit 4 of the primary conveyance path 52 in theconveyance direction of the sheet P, specifically, the upstream sidefrom the conveying roller 16 in the conveyance direction of the sheet P.The reverse conveyance unit 6 includes reverse conveyance rollers 55.

A plurality of pairs of reverse conveyance rollers 55, that is, threepairs of reverse conveyance rollers 55 are provided below the sheetfeeding unit 3, at intervals in the front-rear direction.

In a case of forming images on both sides of a sheet P, the sheet Ppasses through the fixing unit 23, and after the rear end portion of thesheet P passes the flapper 47 positioned at the sheet dischargeposition, the sheet P is conveyed toward the sheet discharge tray 51 andthen is returned into the main body casing 2. Thereafter, the sheet Ppasses the flapper 47 positioned in the re-conveyance position, and isconveyed from the rear side toward the front side in the reverseconveyance unit 6 by the plurality of reverse conveyance rollers 55.

Thereafter, the sheet P having passed the plurality of reverseconveyance rollers 55 is conveyed upward from the front side of thesheet feeding tray 12 and is conveyed into the primary conveyance path52. The sheet P having been conveyed into the primary conveyance path 52is re-conveyed toward the image forming unit 4 by the conveying roller16, and an image is formed on a side having an image not formed yet, andthe sheet P is discharged onto the sheet discharge tray 51.

A path in which a sheet P having been switched by the switchback roller50 is conveyed from the sheet discharge unit 5 toward the reverseconveyance unit 6, and joins the primary conveyance path 52 by thereverse conveyance unit 6 is referred to as a secondary conveyance path56.

2. Main Motor and Process Motor

The printer 1 further includes as an example of a driving source, a mainmotor 68 and a process motor 69 inside the main body casing 2.

The main motor 68 is positioned at a rear portion on the left side atthe substantial center of the main body casing 2 in the verticaldirection. The main motor 68 is configured to generate one-directionrotational driving force when driven. As shown in FIG. 2A, the mainmotor 68 is configured to transmit the rotational driving force to theintermediate sheet discharge roller 48, the switchback roller 50, thesheet feeding roller 14, the conveying roller 16, the registrationroller 17, the black developing roller 31K, the heating roller 43, andthe reverse conveyance rollers 55, respectively. The main motor 68 isconfigured to generate driving force for reversal rotation when a sheetP is jammed inside the main body casing 2, thereby rotating theconveying roller 16, the registration roller 17, the reverse conveyancerollers 55, and the like in a reverse direction.

As shown in FIG. 1, the process motor 69 is positioned at thesubstantially center portion on the left side of the main body casing 2in the vertical direction and the front-rear direction. The processmotor 69 is configured to generate one-direction rotational drivingforce when driven. As shown in FIG. 2B, the process motor 69 isconfigured to transmit the rotational driving force to the blackphotosensitive drum 28K, the yellow photosensitive drum 28Y, the magentaphotosensitive drum 28M, the cyan photosensitive drum 28C, the drivingroller 37, the yellow developing roller 31Y, the magenta developingroller 31M, the cyan developing roller 31C, and the belt cleaning roller42, respectively.

3. Configuration of Driving-Force Transmission Mechanism

The printer 1 includes a driving-force transmission mechanism 76 capableof switching the rotation direction of each of the switchback roller 50and the intermediate sheet discharge roller 48 between a normal rotationdirection and a reverse rotation direction, in order to form images onboth sides of a sheet P, that is, one side and the other side of thesheet P.

The normal rotation direction of the switchback roller 50 and theintermediate sheet discharge roller 48 is the rotation direction forconveying a sheet P toward the sheet discharge tray 51 as describedabove, and the reverse rotation direction of the switchback roller 50and the intermediate sheet discharge roller 48 is the rotation directionfor conveying a sheet P from the discharge opening 49 toward the reverseconveyance unit 6 as described above.

Specifically, as shown in FIG. 4, the switchback roller 50 is a drivingroller which is disposed outside the conveyance path, and the normalrotation direction of the switchback roller 50 is a counterclockwisedirection as seen in a left side view. The intermediate sheet dischargeroller 48 is a driving roller which is disposed outside the conveyancepath, and the normal rotation direction of the intermediate sheetdischarge roller 48 is a clockwise direction as seen in a left sideview. As shown in FIG. 9, the reverse rotation direction of theswitchback roller 50 is a clockwise direction as seen in a left sideview, and the reverse rotation direction of the intermediate sheetdischarge roller 48 is a counterclockwise direction as seen in a leftside view.

The rotation directions of each gear in a normal rotation mode and areverse rotation mode (to be described below) are directions indicatedby arrows shown in each drawing, and will not be described here.

Although not shown, the driving-force transmission mechanism 76 ispositioned at a rear portion of the main body casing 2, and includes aninput gear 79, a rotation-direction switchable gear train 82, and theswitching unit 83.

(1) Input Gear

As shown in FIG. 4, the input gear 79 configures a lower portion of thedriving-force transmission mechanism 76.

The input gear 79 is configured to receive the one-direction rotationaldriving force of the main motor 68 through a plurality of gears (notshown) of the inside of the main body casing 2, thereby rotating in aclockwise direction as seen in a left side view. The input gear 79 is atwo-stage gear including a small-diameter gear and a large-diametergear. The small-diameter gear is engaged with a drive gear 98 (to bedescribed below), and the large-diameter gear is engaged with one of theplurality of gears (not shown) of the inside of the main body casing 2.

The rotational driving force which is generated from the main motor 68is transmitted to gears provided at the left end portions of the sheetfeeding roller 14, the conveying roller 16, the registration roller 17,the black developing roller 31K, the heating roller 43, and the reverseconveyance rollers 55, through the plurality of gears (not shown) of themain body casing 2, thereby rotating the sheet feeding roller 14, theconveying roller 16, the registration roller 17, the black developingroller 31K, the heating roller 43, and the reverse conveyance rollers55.

(2) Rotation-Direction Switchable Gear Train

As shown in FIG. 4, the rotation-direction switchable gear train 82configures an upper rear portion of the driving-force transmissionmechanism 76. The rotation-direction switchable gear train 82 receivesthe one-direction rotational driving force of the main motor 68 throughthe input gear 79 and the switching unit 83. The rotation-directionswitchable gear train 82 includes a switchback roller gear 86 which ispositioned at an upper end portion of the rotation-direction switchablegear train 82, an intermediate sheet discharge roller gear 87 which ispositioned at a lower end portion of the rotation-direction switchablegear train 82, and a first intermediate gear 91, a second intermediategear 92, a third intermediate gear 93, a fourth intermediate gear 94,and a fifth intermediate gear 95 which are positioned between theswitchback roller gear 86 and the intermediate sheet discharge rollergear 87.

As shown in FIG. 3, the switchback roller gear 86 is provided at a leftend portion of the switchback roller 50 so as to rotate integrally withthe switchback roller 50. The switchback roller gear 86 is engaged withthe first intermediate gear 91 (to be described below).

As shown in FIG. 4, the first intermediate gear 91 is positioned on thelower rear side of the switchback roller gear 86, and is rotatablysupported with respect to the left wall of the main body casing 2. Thefirst intermediate gear 91 is engaged with the switchback roller gear 86and the second intermediate gear 92 (to be described below).

The second intermediate gear 92 is positioned below the firstintermediate gear 91, and is rotatably supported with respect to theleft wall of the main body casing 2. The second intermediate gear 92 isa two-stage gear including a small-diameter gear and a large-diametergear. The small-diameter gear is engaged with the first intermediategear 91 and the third intermediate gear 93 (to be described below), andthe large-diameter gear is engaged with the fifth intermediate gear 95(to be described below).

The third intermediate gear 93 is positioned on the lower rear side ofthe second intermediate gear 92, and is rotatably supported with respectto the left wall of the main body casing 2. The third intermediate gear93 is a two-stage gear including a small-diameter gear and alarge-diameter gear. The small-diameter gear is engaged with the secondintermediate gear 92, and the large-diameter gear is engaged with thefourth intermediate gear 94 (to be described below).

The fourth intermediate gear 94 is positioned below the thirdintermediate gear 93 and on the upper front side of the intermediatesheet discharge roller gear 87 (to be described below), and is rotatablysupported with respect to the left wall of the main body casing 2. Thefourth intermediate gear 94 is engaged with the third intermediate gear93 and the intermediate sheet discharge roller gear 87 (to be describedbelow). As will be described below in detail, the fourth intermediategear 94 is configured such that the rotational driving force generatedfrom the main motor 68 is transmitted through the switching unit 83.

The fifth intermediate gear 95 is positioned on the lower front side ofthe second intermediate gear 92, and is rotatably supported with respectto the left wall of the main body casing 2. The fifth intermediate gear95 is engaged with the second intermediate gear 92. As will be describedbelow in detail, the fifth intermediate gear 95 is configured such thatthe rotational driving force generated from the main motor 68 istransmitted through the switching unit 83.

The intermediate sheet discharge roller gear 87 is provided at a leftend portion of the intermediate sheet discharge roller 48, so as torotate integrally with the intermediate sheet discharge roller 48. Theintermediate sheet discharge roller gear 87 is engaged with the fourthintermediate gear 94.

(3) Switching Unit

The switching unit 83 configures a portion of the driving-forcetransmission mechanism 76 between the input gear 79 and therotation-direction switchable gear train 82. The switching unit 83includes the drive gear 98, a holder 99, a pendulum gear 100, a sectorgear 101 (an example of a switching gear), a lever 103

(an example of an engaging member), and a solenoid switch 104 (anexample of a switching element).

(3-1) Drive Gear, Holder, and Pendulum Gear

The drive gear 98 is positioned on the upper rear side of the input gear79, and a drive support shaft 108 of the drive gear 98 is supported onthe left wall of the main body casing 2, whereby the drive gear 98 isrotatably supported with respect to the main body casing 2. The drivesupport shaft 108 of the drive gear 98 is inserted through a drive gearshaft insertion hole 113 of the holder 99 (to be described below),whereby the drive gear 98 supports the holder 99 such that the holder 99is rotatable. The drive gear 98 is engaged with the input gear 79 andthe pendulum gear 100 (to be described below).

The holder 99 includes a gear supporting unit 110 and a switching-powerreceiving unit 111. The following description will be made withreference to directions referring to the posture of the holder 99 in thenormal rotation mode, specifically, the following description will bemade with reference to the directions shown in FIG. 4.

The gear supporting unit 110 configures a rear portion of the holder 99,and is formed in a flat plate shape which has a substantiallyrectangular shape as seen in a side view and has substantially a letter“U” shape as seen in a plan view, so as to sandwich the pendulum gear100 (to be described below) from both outer sides in the left-rightdirection, as shown in FIGS. 4 and 6. As shown in FIG. 4, the gearsupporting unit 110 includes the drive gear shaft insertion hole 113 anda pendulum gear shaft insertion hole 114.

The drive gear shaft insertion hole 113 is formed in the left-rightdirection on the lower side of the substantially center portion of thegear supporting unit 110 in the front-rear direction such that the drivesupport shaft 108 of the drive gear 98 can be inserted therethrough.

The pendulum gear shaft insertion hole 114 is formed in the left-rightdirection at an upper rear end portion of the gear supporting unit 110such that a pendulum gear shaft 120 of the pendulum gear 100 can beinserted therethrough.

The switching-power receiving unit 111 configures a front portion of theholder 99. The switching-power receiving unit 111 includes a frameportion 116 and a cover portion 117.

As shown in FIG. 5, the frame portion 116 extends continuously from afront end portion of the gear supporting unit 110 toward the front side,and has substantially a rectangular frame shape having a hole formed inthe left-right direction, as seen in a side view.

As shown in FIG. 4, the cover portion 117 is formed in a thin plateshape to close the left end portion of the frame portion 116. The coverportion 117 has a long hole 118 and a hook 119.

The long hole 118 is formed in the left-right direction from an upperrear end portion of the cover portion 117 to a substantially centerportion of the cover portion 117 in the front-rear direction as seen ina side view, along an arc having a center at the drive gear shaftinsertion hole 113 of the gear supporting unit 110.

The hook 119 is positioned at a portion of the cover portion 117 at thefront side of the long hole 118. As shown in FIG. 3, the hook 119 hassubstantially a claw shape protruding from the left surface of the coverportion 117 toward the left side and bent toward the lower front side.

The pendulum gear shaft 120 of the pendulum gear 100 is supported in thependulum gear shaft insertion hole 114 of the holder 99, whereby thependulum gear 100 is rotatably supported with respect to the holder 99.The pendulum gear 100 is always engaged with the drive gear 98. Thependulum gear 100 is configured to be selectively engaged with thefourth intermediate gear 94 or the fifth intermediate gear 95, byswinging of the holder 99 around the drive support shaft 108.

Specifically, as shown in FIG. 4, if the holder 99 is rotated around thedrive support shaft 108 in a counterclockwise direction as seen in aleft side view, the pendulum gear 100 is positioned at a firstengagement position where the pendulum gear 100 is engaged with thefourth intermediate gear 94 from the front side. Therefore, theone-direction rotational driving force of the main motor 68 istransmitted to the switchback roller 50 through the input gear 79, thedrive gear 98, the pendulum gear 100, the fourth intermediate gear 94,the third intermediate gear 93, the second intermediate gear 92, thefirst intermediate gear 91, and the switchback roller gear 86. As aresult, the switchback roller 50 rotates in the normal rotationdirection. Also, the one-direction rotational driving force of the mainmotor 68 is transmitted to the intermediate sheet discharge roller 48through the input gear 79, the drive gear 98, the pendulum gear 100, thefourth intermediate gear 94, and the intermediate sheet discharge rollergear 87. As a result, the intermediate sheet discharge roller 48 rotatesin the normal rotation direction.

The gear arrangement of the fourth intermediate gear 94, the thirdintermediate gear 93, the second intermediate gear 92, the firstintermediate gear 91, and the switchback roller gear 86 which transmitthe rotational driving force from the pendulum gear 100 for rotating theswitchback roller 50 in the normal rotation direction in a case wherethe holder 99 is rotated in a counterclockwise direction as seen in aleft side view is considered as an example of a first gear train. Astate in which the pendulum gear 100 of the switching unit 83 is held atthe first engagement position where the pendulum gear 100 is engagedwith the first gear train, such that the one-direction rotationaldriving force of the main motor 68 is transmitted to the first geartrain and the switchback roller 50 and the intermediate sheet dischargeroller 48 rotate in their normal rotation directions is referred to as anormal rotation mode (an example of a first mode of the switching unit83).

Also, if the holder 99 is rotated around the drive support shaft 108 ina clockwise direction as seen in a left side view, the pendulum gear 100is positioned at a second engagement position where the pendulum gear isengaged with the fifth intermediate gear 95 from the lower side, asshown in FIG. 9. As a result, the one-direction rotational driving forceof the main motor 68 is transmitted to the switchback roller 50 throughthe input gear 79, the drive gear 98, the pendulum gear 100, the fifthintermediate gear 95, the second intermediate gear 92, the firstintermediate gear 91, and the switchback roller gear 86. As a result,the switchback roller 50 rotates in the reverse rotation direction.Also, the one-direction rotational driving force of the main motor 68 istransmitted to the intermediate sheet discharge roller 48 through theinput gear 79, the drive gear 98, the pendulum gear 100, the fifthintermediate gear 95, the second intermediate gear 92, the thirdintermediate gear 93, the fourth intermediate gear 94, and theintermediate sheet discharge roller gear 87. As a result, theintermediate sheet discharge roller 48 rotates in the reverse rotationdirection.

The gear arrangement of the fifth intermediate gear 95, the secondintermediate gear 92, the first intermediate gear 91, and the switchbackroller gear 86 which transmit the rotational driving force from thependulum gear 100 for rotating the switchback roller 50 in the reverserotation direction in a case where the holder 99 is rotated in aclockwise direction as seen in a left side view is considered as anexample of a second gear train. A state in which the pendulum gear 100of the switching unit 83 is held at the second engagement position wherethe pendulum gear 100 is engaged with the second gear train, such thatthe one-direction rotational driving force of the main motor 68 istransmitted to the second gear train and the switchback roller 50 andthe intermediate sheet discharge roller 48 rotate in their reverserotation directions is referred to as a reverse rotation mode (anexample of a second mode of the switching unit 83).

The pendulum gear 100 is rotated around the drive support shaft 108 soas to be positioned at a middle position between the fourth intermediategear 94 and the fifth intermediate gear 95, as shown in FIG. 13, therebybeing positioned at a disengagement position where the pendulum gear 100is not engaged with any of the fourth intermediate gear 94 and the fifthintermediate gear 95. A state in which the pendulum gear 100 of theswitching unit 83 is held at the disengagement position where thependulum gear 100 is not engaged with any of the first gear train andthe second gear train, such that the one-direction rotational drivingforce of the main motor 68 is not transmitted to any of the first geartrain and the second gear train and the switchback roller 50 and theintermediate sheet discharge roller 48 do not rotate is referred to as astack mode (an example of a third mode of the switching unit 83).

Further, in the main body casing 2, a tension spring 121 is positionedso as to connect the hook 119 of the holder 99, and a hook (not shown)which is provided at a portion of the holder 99 on the rear side fromthe hook 119.

Therefore, the holder 99 is always biased by the biasing force of thetension spring 121, such that the holder 99 rotates around the drivesupport shaft 108 in a counterclockwise direction, that is, the pendulumgear 100 is positioned at the first engagement position where thependulum gear is engaged with the fourth intermediate gear 94, as shownin FIG. 4.

(3-2) Sector Gear

The sector gear 101 is positioned on the upper front side of the drivegear 98, and is rotatably supported with respect to the left wall of themain body casing 2. As shown in FIGS. 8A to 8D, the sector gear 101includes a sector gear shaft 125, a first partition plate 126, apartially toothed gear 130, a cylindrical unit 131 (an example of aregulating member), a V-shaped cam 145 (an example of a second cam), asecond partition plate 127, and an I-shaped cam 146 (an example of afirst cam). The following description will be made with reference todirections referring to the posture of the sector gear 101 in the normalrotation mode, specifically, the following description will be made withreference to the directions shown in FIGS. 8A to 8D.

The sector gear shaft 125 is formed at a center portion of the sectorgear 101 as seen in a side view so as to extend in a substantiallycolumnar shape in the left-right direction. As shown in FIG. 4, the leftend portion of the sector gear shaft 125 is inserted through the longhole 118 of the holder 99.

As shown in FIGS. 8A to 8D, the first partition plate 126 is at asubstantially center portion of the sector gear shaft 125 in theleft-right direction and has a flat plate shape having a substantiallycircular shape having a diameter larger than the diameter of the sectorgear shaft 125, as seen in a side view.

The partially toothed gear 130 has a substantially cylindrical shapeextending from the right surface of the first partition plate 126 towardthe right side. The partially toothed gear 130 has non-tooth portions133 and toothed portions 134.

The non-tooth portions 133 include a first non-tooth portion 135 whichis in a range of about 45° on the outer periphery of a lower rearportion of the partially toothed gear 130 and has no gear teeth, and asecond non-tooth portion 136 which is at a position deviated in aclockwise direction from the first non-tooth portion 135 by about 90° asseen in a right side view, that is, in a range of about 90° on the outerperiphery of a front portion of the partially toothed gear 130 and hasgear teeth.

The toothed portions 134 are portions where gear teeth are formed,except for the non-tooth portions 133 of the partially toothed gear 130.Specifically, the toothed portions 134 include a first toothed portion137 which is adjacent to the first non-tooth portion 135 in a clockwisedirection as seen in the right side view of the first non-tooth portion135, and is adjacent to the second non-tooth portion 136 in acounterclockwise direction as seen in the right side view of the secondnon-tooth portion 136, and a second toothed portion 138 which isadjacent to the second non-tooth portion 136 in a clockwise direction asseen in the right side view of the second non-tooth portion 136, and isadjacent to the first non-tooth portion 135 in a counterclockwisedirection as seen in the right side view of the first non-tooth portion135.

The cylindrical unit 131 is formed in a substantially cylindrical shapeextending from the right surface of the first partition plate 126positioned on the inner side than the partially toothed gear 130, towardthe right side, as seen in a side view. The cylindrical unit 131 has adiameter smaller than that of the partially toothed gear 130, and ispositioned such that the right end portion of the cylindrical unit 131is positioned between the right end portion of the partially toothedgear 130 and the right end portion of the sector gear shaft 125. Thecylindrical unit 131 has protruding portions 140.

The protruding portions 140 include a first protruding portion 141 whichis at an upper rear portion of the outer periphery of the cylindricalunit 131 and protrudes toward the outside in a radial direction, and asecond protruding portion 142 which is at a lower portion of the outerperiphery of the cylindrical unit 131 and protrudes toward the outsidein the radial direction.

The first protruding portion 141 has a substantially triangular shapeprotruding from the outer circumferential surface of the cylindricalunit 131 toward the outside in the radial direction of the cylindricalunit 131, as seen in a side view. A surface of the first protrudingportion 141 extending along the radial direction of the cylindrical unit131 is defined as a first engagement surface 143. The first engagementsurface 143 is a surface of the cylindrical unit 131, which faces acounterclockwise direction in a circumferential direction as seen in aright side view. The first protruding portion 141 is formed from theright end portion of the partially toothed gear 130 to the right endportion of the cylindrical unit 131 as seen in a front view, such thatthe tip end of the first protruding portion overlaps a portion of thesecond toothed portion 138 of the partially toothed gear 130 as seen ina side view.

The second protruding portion 142 is at a position of the outerperiphery of the cylindrical unit 131 deviated from the first protrudingportion 141 in a clockwise direction by 150° as seen in a right sideview, and has a substantially triangular shape protruding from the outercircumferential surface of the cylindrical unit 131 toward the outsidein the radial direction of the cylindrical unit 131, as seen in a sideview. A surface of the second protruding portion 142 extending along theradial direction of the cylindrical unit 131 is defined as a secondengagement surface 144. The second engagement surface 144 is a surfaceof the cylindrical unit 131, which faces a counterclockwise direction ina circumferential direction as seen in a right side view. The secondprotruding portion 142 is formed over a range from the right end portionof the partially toothed gear 130 to a middle position between the rightend portion of the partially toothed gear 130 and the right end portionof the cylindrical unit 131 as seen in a front view, such that the tipend of the second protruding portion overlaps a portion of the firsttoothed portion 137 of the partially toothed gear 130 as seen in a sideview. That is, the first protruding portion 141 has a portion whichoverlaps the second protruding portion 142 when the first protrudingportion 141 is projected in the circumferential direction of thecylindrical unit 131, and a portion which does not overlap the secondprotruding portion 142 when the first protruding portion 141 isprojected in the circumferential direction of the cylindrical unit 131

The V-shaped cam 145 extends from the left surface of the firstpartition plate 126 toward the left side. As shown in FIG. 6, theV-shaped cam 145 is formed in a substantial rod shape havingsubstantially a V shape extending in a radial direction from the outercircumferential surface of the sector gear shaft 125 as seen in a sideview. Specifically, the V-shaped cam 145 is formed such that one endportion of the V-shaped cam extends from the sector gear shaft 125toward the second non-tooth portion 136, and the other end portion ofthe V-shaped cam extends from the sector gear shaft 125 toward thesecond toothed portion 138. The tip ends of the one end portion and theother end portion of the V-shaped cam 145 have substantially circularshapes as seen in a side view.

The second partition plate 127 is positioned on the left side of theV-shaped cam 145 with a gap in a left direction from the first partitionplate 126, and the right surface of the second partition plate 127 isconnected to the V-shaped cam 145. The second partition plate 127 has asubstantially flat plate shape larger than the diameter of the sectorgear shaft 125. Specifically, the second partition plate 127 has asubstantially triangular shape having the sector gear shaft 125, one endportion of the V-shaped cam 145, and the vicinity of the other endportion of the V-shaped cam 145 as vertexes as seen in a side view. Eachof the vertexes of the second partition plate 127 has a substantiallysemi-circular shape as seen in a side view. The second partition plate127 has such a size that the second partition plate 127 falls in thefirst partition plate 126 and the V-shaped cam 145 falls in the secondpartition plate 127.

The I-shaped cam 146 is formed on the left side from the secondpartition plate 127, in a substantial rod shape extending from the outercircumferential surface of the sector gear shaft 125 toward the outsidein a radial direction. The right surface of the I-shaped cam 146 isconnected to the second partition plate 127. The I-shaped cam 146extends toward the upper front vertex of the second partition plate 127.That is, the I-shaped cam 146 overlaps one end portion of the V-shapedcam 145 extending toward the second non-tooth portion 136 when projectedin the left-right direction. The I-shaped cam 146 extends in a directionof about two o'clock from the sector gear shaft 125 as seen in a leftside view, in the normal rotation mode (to be described below). The tipend of the I-shaped cam 146 has a substantially circular shape as seenin a side view.

The second partition plate 127, the V-shaped cam 145, and the I-shapedcam 146 are configured as a cam 147.

(3-3) Lever and Solenoid Switch

As shown in FIG. 4, the lever 103 is positioned on the upper front sideof the sector gear 101, and is supported so as to be able to swing withrespect to the left wall of the main body casing 2. As shown in FIGS. 7Aand 7B, the lever 103 includes a lever shaft 151, a connection portion152, a first engagement portion 153, and a second engagement portion154. The following description will be made with reference to directionsreferring to the state of the lever 103 in the normal rotation mode,specifically, the following description will be made with reference tothe directions shown in FIGS. 7A and 7B.

The lever shaft 151 is formed in a substantially cylindrical shapeextending in the left-right direction.

The connection portion 152 is formed in a substantial claw shapeprotruding from the outer circumferential surface of an upper portion ofthe lever shaft 151 toward the upper rear side, and a hook portion 165of the solenoid switch 104 (to be described below) is fit therein.

The first engagement portion 153 has a shape protruding from the outercircumferential surface of a lower rear portion of the lever shaft 151toward the lower rear side. The first engagement portion 153 has a firstengaging claw 158.

The first engaging claw 158 configures a lower rear end portion of thefirst engagement portion 153, and is formed in a substantially prismaticshape having a substantially rectangular shape as seen in a side view.The first engaging claw 158 is disposed so as to overlap the firstprotruding portion 141 and the second protruding portion 142 in theaxial direction of the cylindrical unit 131, that is, in the left-rightdirection. In other words, the first engaging claw 158 is disposed so asto overlap the first protruding portion 141 and the second protrudingportion 142 when the cylindrical unit 131 is projected in thecircumferential direction.

The second engagement portion 154 is formed to protrude from the outercircumferential surface of a lower front portion of the lever shaft 151toward the lower front side. The second engagement portion 154 has asecond engaging claw 159.

The second engaging claw 159 configures a lower front end portion of thesecond engagement portion 154, and is formed in a claw shape bent towardthe rear side. The second engaging claw 159 is disposed so as not tooverlap the second protruding portion 142 and so as to overlap the firstprotruding portion 141, in the axial direction of the cylindrical unit131, that is, in the left-right direction. In other words, the secondengaging claw 159 is disposed such that the second engaging claw doesnot overlap the second protruding portion 142 and overlaps the firstprotruding portion 141 when the cylindrical unit 131 is projected in thecircumferential direction.

Further, a shaft (not shown) of the main body casing 2 on the upperfront side of the sector gear 101 is inserted through the lever shaft151, whereby the lever 103 is supported to be able to swing with respectto the left wall of the main body casing 2. The lever 103 can swingbetween a first engagement position where the first engaging claw 158 isclose to the cylindrical unit 131 of the sector gear 101 and the secondengaging claw 159 is separated from the cylindrical unit 131 of thesector gear 101, as shown in FIGS. 7A, 7B, 16A, and 16B, and a secondengagement position where the first engaging claw 158 is relativelyseparated from the cylindrical unit 131 of the sector gear 101, and thesecond engaging claw 159 is relatively close to the cylindrical unit 131of the sector gear 101, as shown in FIGS. 12A and 12B.

That is, the lever 103 can move between the first engagement positionwhere the first engagement portion 153 can be engaged with the firstprotruding portion 141 and the second protruding portion 142, and thesecond engagement portion 154 is not engaged with the first protrudingportion 141 and the second protruding portion 142, and the secondengagement position where the second engagement portion 154 can beengaged with the first protruding portion 141, and the first engagementportion 153 is not engaged with the first protruding portion 141 and thesecond protruding portion 142.

The solenoid switch 104 is positioned on the lever 103 as shown in FIG.4 so as to switch the lever 103 between the first engagement positionshown in FIGS. 7A, 7B, 16A, and 16B and the second engagement positionshown in FIGS. 12A and 12B, and is fixed with respect to the left wallof the main body casing 2. The solenoid switch 104 receives a signalfrom a CPU 72 (to be described below), thereby being switched between anexcited state (an example of a second state) in which a current flows,and a non-excited state (an example of a first state) in which nocurrent flows. The solenoid switch 104 includes a main body portion 163,and an advance/retreat portion 164.

The main body portion 163 is formed in a substantial box shape having anopen lower side, and includes an electromagnet (not shown) and acompression spring (not shown) therein.

The advance/retreat portion 164 is formed in a substantially cylindricalshape protruding downward from the open portion of the main body portion163. The advance/retreat portion 164 has the hook portion 165.

The hook portion 165 is at the lower end portion of the advance/retreatportion 164, and has a groove shape depressed from the circumferentialsurface of the advance/retreat portion 164 toward the center of theadvance/retreat portion 164. The hook portion 165 is fit with respect tothe connection portion 152 of the lever 103.

When the solenoid switch 104 is in the non-excited state, theadvance/retreat portion 164 advances by biasing force of the compressionspring (not shown) of the inside of the main body portion 163 such thatthe hook portion 165 is relatively separated from the main body portion163, whereby the lever 103 is held at the first engagement positionshown in FIGS. 7A, 7B, 16A, and 16B. When the solenoid switch 104 is inthe excited state, a current flows in the electromagnet (not shown) ofthe inside of the main body portion 163, whereby the electromagnet ismagnetized, and the upper portion of the advance/retreat portion 164 ispulled further toward the upper side of the main body portion 163 by themagnetic force, whereby the advance/retreat portion 164 retreats againstthe biasing force of the compression spring (not shown) of the inside ofthe main body portion 163 such that the hook portion 165 relativelyapproaches the main body portion 163, whereby the lever 103 is held atthe second engagement position shown in FIGS. 12A and 12B.

As shown in FIG. 6, the switching unit 83 includes a torsion spring 148(an example of a biasing member) for biasing the V-shaped cam 145 of thesector gear 101 from the upper front side toward the lower rear side. Asa result, the torsion spring 148 biases the sector gear 101 by itsbiasing force such that the sector gear 101 rotates in a clockwisedirection as seen in a left side view.

Meanwhile, the solenoid switch 104 is set to the non-excited state orthe excited state, whereby the lever 103 is positioned at the firstengagement position or the second engagement position such that thefirst engaging claw 158 of the first engagement portion 153 or thesecond engaging claw 159 of the second engagement portion 154 is engagedwith the protruding portion 140 of the cylindrical unit 131, whereby thelever 103 and the solenoid switch 104 regulate rotation of theabove-described sector gear 101 by the biasing force of the torsionspring 148 in a clockwise direction as seen in a left side view. Whenrotation of the sector gear 101 is regulated, a non-tooth portion 133(the first non-tooth portion 135 or the second non-tooth portion 136) ofthe partially toothed gear 130 faces the drive gear 98.

Accordingly, the above-described sector gear 101 is configured such thateven though the drive gear 98 always rotates, the toothed portion 134 ofthe partially toothed gear 130 is engaged with the drive gear 98,thereby receiving the one-direction rotational driving force of the mainmotor 68, and is configured to cause the non-tooth portion 133 of thepartially toothed gear 130 face the drive gear 98 such that the drivingforce generated from the main motor 68 is not transmitted.

Specifically, in the normal rotation mode, while the torsion spring 148biases one end portion of the V-shaped cam 145 from the upper front sidetoward the lower rear side as shown in FIG. 6, thereby rotating thesector gear 101 in a clockwise direction as seen in a left side view,the solenoid switch 104 is set to the non-excited state and the lever103 is positioned at the first engagement position such that the firstengaging claw 158 of the first engagement portion 153 is engaged withthe first engagement surface 143 of the first protruding portion 141 ofthe cylindrical unit 131, as shown in FIGS. 7A and 7B, whereby the lever103 and the solenoid switch 104 hold the sector gear 101 against thebiasing force of the torsion spring 148 such that the first non-toothportion 135 of the partially toothed gear 130 faces the drive gear 98 asshown in FIG. 4.

In the reverse rotation mode, while the torsion spring 148 biases theother end portion of the V-shaped cam 145 from the upper front sidetoward the lower rear side as shown in FIG. 11, thereby rotating thesector gear 101 in a clockwise direction as seen in a left side view,the solenoid switch 104 is set to the excited state and the lever 103 ispositioned at the second engagement position such that the secondengaging claw 159 of the second engagement portion 154 is engaged withthe first engagement surface 143 of the first protruding portion 141 ofthe cylindrical unit 131, as shown in FIGS. 12A and 12B, whereby thelever 103 and the solenoid switch 104 hold the sector gear 101 againstthe biasing force of the torsion spring 148 such that a portion of thesecond non-tooth portion 136 of the partially toothed gear 130 on thedownstream side in the rotation direction faces the drive gear 98 asshown in FIG. 9.

In the stack mode, while the torsion spring 148 biases the other endportion of the V-shaped cam 145 from the upper front side toward thelower rear side as shown in FIG. 15, thereby rotating the sector gear101 in a clockwise direction as seen in a left side view, the solenoidswitch 104 is set to the excited state and the lever 103 is positionedat the first engagement position such that the first engaging claw 158of the first engagement portion 153 is engaged with the secondengagement surface 144 of the second protruding portion 142 of thecylindrical unit 131, as shown in FIGS. 16A and 16B, whereby the lever103 and the solenoid switch 104 hold the sector gear 101 against thebiasing force of the torsion spring 148 such that a portion of thesecond non-tooth portion 136 of the partially toothed gear 130 on theupstream side in the rotation direction faces the drive gear 98 as shownin FIG. 13.

The lever 103, the solenoid switch 104, and the cylindrical unit 131 ofthe sector gear 101 are configured as an example of a locking unit.

4. Mode Switching Operation of Switching Unit

As described above, the switching unit 83 switches the solenoid switch104 between the excited state and the non-excited state, therebyperforming switching among the normal rotation mode, the reverserotation mode, and the stack mode.

The following description will be made on the assumption that the mainmotor 68 is always driven, whereby the input gear 79 is rotated in onedirection.

(1) Switching Operation from Normal Rotation Mode to Reverse RotationMode

Subsequently, a switching operation from the normal rotation mode to thereverse rotation mode will be described.

In order to switch the switching unit 83 from the normal rotation modeto the reverse rotation mode, the solenoid switch 104 is switched fromthe non-excited state in the normal rotation mode as shown in FIG. 4 tothe excited state as shown in FIG. 9.

As a result, the lever 103 swings in a clockwise direction as seen in aleft side view, thereby moving from the first engagement position to thesecond engagement position.

As a result, contact of the first engaging claw 158 and the firstengagement surface 143 of the first protruding portion 141 is released,and as shown in FIG. 11, the sector gear 101 rotates in a clockwisedirection as seen in a left side view by the biasing force of thetorsion spring 148 on one end portion of the V-shaped cam 145.

If the sector gear 101 rotates, the first toothed portion 137 of thepartially toothed gear 130 moves to a position facing the drive gear 98.As a result, the first toothed portion 137 is engaged with the drivegear 98, and the sector gear 101 rotates with rotation of the drive gear98.

At this time, the I-shaped cam 146 rotates with rotation of the sectorgear 101 as shown in FIG. 10. As the first toothed portion 137 isengaged with the drive gear 98, the I-shaped cam 146 is rotated in aclockwise direction as seen in a left side view and comes into contactwith the frame portion 116 of the holder 99 from the upper side. Thesector gear 101 keeps rotating even after the I-shaped cam 146 and theframe portion 116 have come into contact with each other, and thus theI-shaped cam 146 rotates while pressing the frame portion 116 downward.

If the frame portion 116 of the holder 99 is pressed downward, theholder 99 rotates around the drive support shaft 108 in a clockwisedirection as seen in a left side view. The rotation of holder 99 causesthe pendulum gear 100 pivotally supported on the holder 99 to move fromthe first engagement position toward the second engagement position.Also, if the pendulum gear 100 moves to the second engagement position,the I-shaped cam 146 becomes a state in which the I-shaped cam extendsin a direction of about six o'clock as seen in a left side view andpresses the frame portion 116 such that the frame portion 116 is thelowest.

While the sector gear 101 rotates such that the pendulum gear 100 movesto the second engagement position, the second non-tooth portion 136faces the drive gear 98 as shown in FIG. 11. At this time, the torsionspring 148 applies biasing force for rotating the sector gear 101 in aclockwise direction as seen in a left side view, to the other endportion of the V-shaped cam 145.

If the sector gear 101 rotates by biasing of the torsion spring 148 onthe other end portion of the V-shaped cam 145, the second engaging claw159 of the lever 103 positioned at the second engagement position comesinto contact with the first protruding portion 141 of the sector gear101, as shown in FIGS. 12A and 12B.

As a result, the rotation of the sector gear 101 is regulated, and theswitching unit 83 is switched from the normal rotation mode to thereverse rotation mode.

(2) Switching Operation from Reverse Rotation Mode to Stack Mode

Subsequently, a switching operation from the reverse rotation mode tothe stack mode will be described.

In order to switch the switching unit 83 from the reverse rotation modeto the stack mode, the solenoid switch 104 is switched from the excitedstate in the reverse rotation mode as shown in FIG. 9 to the non-excitedstate as shown in FIG. 13.

As a result, the lever 103 swings in a counterclockwise direction asseen in a left side view, thereby moving from the second engagementposition to the first engagement position.

As a result, contact of the second engaging claw 159 and the firstengagement surface 143 of the first protruding portion 141 is released,and the sector gear 101 rotates in a clockwise direction as seen in aleft side view, by the biasing force of the torsion spring 148 on theother end portion of the V-shaped cam 145, as shown in FIG. 15.

At this time, the I-shaped cam 146 rotates with rotation of the sectorgear 101. The I-shaped cam 146 rotates from the position of about sixo'clock as seen in a left side view, in a clockwise direction as seen ina left side view. Since the pressing position of the I-shaped cam 146 onthe frame portion 116 moves upward, the holder 99 rotates around thedrive support shaft 108 in a counterclockwise direction as seen in aleft side view, by upward biasing force of the tension spring 121 asshown in FIG. 13.

The rotation of the holder 99 causes the pendulum gear 100 pivotallysupported on the holder 99 to move from the second engagement positiontoward the first engagement position.

Further, while the sector gear 101 rotates by the biasing force of thetorsion spring 148, as shown in FIGS. 16A and 16B, the second protrudingportion 142 of the sector gear 101 comes into contact with the firstengaging claw 158 of the lever 103 positioned at the first engagementposition, from the rear side.

As a result, rotation of the sector gear 101 is regulated, and rotationof the sector gear 101 by the biasing force of the torsion spring 148 isregulated.

At this time, as shown in FIG. 14, the I-shaped cam 146 of the sectorgear 101 is directed to about seven o'clock with respect to the sectorgear shaft 125 as seen in a left side view. Since the rotation of thesector gear 101 is regulated in a state where the I-shaped cam 146presses the frame portion 116, the pendulum gear 100 is held at thedisengagement position where the pendulum gear is not engaged with anyof the first gear train and the second gear train.

As a result, the switching unit 83 is switched from the reverse rotationmode to the stack mode.

(3) Switching Operation from Stack Mode to Normal Rotation Mode

A switching operation from the stack mode to the normal rotation modewill be described.

In order to switch the switching unit 83 from the stack mode to thenormal rotation mode, the solenoid switch 104 in the stack mode isswitched from the non-excited state to the excited state, and then isswitched to the non-excited state again.

Therefore, the lever 103 swings in a clockwise direction as seen in aleft side view, thereby moving from the first engagement position to thesecond engagement position, and then immediately swings in acounterclockwise direction as seen in a left side view, thereby movingfrom the second engagement position to the first engagement position.

As a result, contact of the first engaging claw 158 and the secondengagement surface 144 of the second protruding portion 142 is released,and the sector gear 101 rotates in a clockwise direction as seen in aleft side view, by the biasing force of the torsion spring 148 on theother end portion of the V-shaped cam 145, as shown in FIG. 6. Accordingto the rotation of the sector gear 101, the second protruding portion142 moves from a position where the second protruding portion 142 isengaged with the first engaging claw 158, and then the first engagingclaw 158 is moved to the first engagement position again.

If the sector gear 101 rotates, the second toothed portion 138 of thepartially toothed gear 130 moves a position where the second toothedportion 138 faces the drive gear 98. As a result, the second toothedportion 138 is engaged with the drive gear 98, and with rotation of thedrive gear 98, the sector gear 101 rotates.

At this time, the I-shaped cam 146 rotates with rotation of the sectorgear 101. The I-shaped cam 146 rotates from the position of about seveno'clock, in a clockwise direction as seen in a left side view, therebybeing separated from the frame portion 116. If the I-shaped cam 146 isseparated from the frame portion 116, the holder 99 rotates around thedrive support shaft 108 in a counterclockwise direction as seen in aleft side view, by the upward biasing force of the tension spring 121.The rotation of the holder 99 causes the pendulum gear 100 pivotallysupported on the holder 99 to move from the disengagement positiontoward the first engagement position.

If the sector gear 101 rotates, the first non-tooth portion 135 facesthe drive gear 98. At this time, the torsion spring 148 applies thebiasing force to one end portion of the V-shaped cam 145, for rotatingthe sector gear 101 in a clockwise direction as seen in a left sideview.

If the sector gear 101 rotates by biasing of the torsion spring 148 onone end portion of the V-shaped cam 145, the first engaging claw 158 ofthe lever 103 positioned at the first engagement position comes intocontact with the first protruding portion 141 of the sector gear 101, asshown in FIGS. 7A and 7B.

As a result, the rotation of the sector gear 101 is regulated, and theswitching unit 83 is switched from the stack mode to the normal rotationmode.

(4) Switching Operation from Stack Mode to Reverse Rotation Mode

Subsequently, a switching operation from the stack mode to the reverserotation mode will be described.

In order to switch the switching unit 83 from the stack mode to thereverse rotation mode, the solenoid switch 104 in the stack mode isswitched from the non-excited state as shown in FIG. 13 to the excitedstate as shown in FIG. 9, and is held in the excited state for apredetermined time period or more.

As a result, the lever 103 is swung in a clockwise direction as seen ina left side view, and is held in a state where the lever has been movedfrom the first engagement position to the second engagement position.

Then, contact of the first engaging claw 158 and the second engagementsurface 144 of the second protruding portion 142 is released, and thesector gear 101 rotates in a clockwise direction as seen in a left sideview, by the biasing force of the torsion spring 148 on the other endportion of the V-shaped cam 145, as shown in FIG. 6.

If the sector gear 101 rotates, the second toothed portion 138 of thepartially toothed gear 130 is engaged with the drive gear 98, and withrotation of the drive gear 98, the sector gear 101 rotates.

At this time, the I-shaped cam 146 rotates with rotation of the sectorgear 101 as shown in FIG. 5. The I-shaped cam 146 rotates from theposition of about seven o'clock as seen in a left side view, in aclockwise direction as seen in a left side view, thereby being separatedfrom the frame portion 116. If the I-shaped cam 146 is separated fromthe frame portion 116, the holder 99 is rotated around the drive supportshaft 108 in a counterclockwise direction as seen in a left side view,by the upward biasing force of the tension spring 121. The rotation ofthe holder 99 causes the pendulum gear 100 pivotally supported on theholder 99 to move from the disengagement position toward the firstengagement position.

If the sector gear 101 rotates, the first non-tooth portion 135 facesthe drive gear 98 as shown in FIG. 6. At this time, the torsion spring148 applies the biasing force to one end portion of the V-shaped cam145, for rotating the sector gear 101 in a clockwise direction as seenin a left side view.

While the sector gear 101 rotates by biasing of the torsion spring 148on one end portion of the V-shaped cam 145, the second protrudingportion 142 of the sector gear 101 comes close to the second engagingclaw 159 of the lever 103 positioned at the second engagement position.However, since the second engaging claw 159 and the second protrudingportion 142 are at positions where they are deviated (do not overlap) inthe left-right direction which is the axial direction of the cylindricalunit 131, the sector gear 101 keeps rotating, without engaging betweenthe second engaging claw 159 and the second protruding portion 142.

Thereafter, although the pendulum gear 100 moves to the first engagementposition by rotation of the sector gear 101, since the first engagingclaw 158 is separated from the cylindrical unit 131, the sector gear 101keeps rotating.

Therefore, the pendulum gear 100 is swung toward the second engagementposition, without being held at the first engagement position.

The process after the pendulum gear 100 is swung from the firstengagement position toward the second engagement position is the same asthe switching operation from the normal rotation mode to the reverserotation mode, and thus will not be described.

Accordingly, the rotation of the sector gear 101 is regulated, and theswitching unit 83 is switched from the stack mode to the reverserotation mode is performed.

5. Effects of Driving-Force Transmission Mechanism

(1) According to the printer 1, the switching unit 83 has the normalrotation mode in which the switching unit 83 holds the pendulum gear 100at the first engagement position where the pendulum gear 100 is engagedwith the fourth intermediate gear 94 as shown in FIG. 4 and transmitsthe one-direction rotational driving force of the main motor 68 to thefirst gear train, thereby setting the rotation direction of theswitchback roller 50 and the intermediate sheet discharge roller 48 intothe normal rotation direction, the reverse rotation mode in which theswitching unit 83 holds the pendulum gear 100 at the second engagementposition where the pendulum gear 100 is engaged with the fifthintermediate gear 95 as shown in FIG. 9 and transmits the one-directionrotational driving force of the main motor 68 to the second gear train,thereby setting the rotation direction of the switchback roller 50 andthe intermediate sheet discharge roller 48 into the reverse rotationdirection, and the stack mode in which the switching unit 83 holds thependulum gear 100 at the disengagement position between the fourthintermediate gear 94 and the fifth intermediate gear 95 as shown in FIG.13, such that the one-direction rotational driving force of the mainmotor 68 is not transmitted to any of the first gear train and thesecond gear train, and thus the switchback roller 50 and theintermediate sheet discharge roller 48 do not rotate.

Accordingly, it is not necessary to switch the rotational driving forceof the main motor 68 among the normal rotation direction, the reverserotation direction and stop rotation in order to switch the rotationdirection of the switchback roller 50 or stopping the switchback roller50. Therefore, it is possible to use the main motor 68 not only as amotor for generating rotational driving force for rotating rotary bodies(the sheet feeding roller 14, the conveying roller 16, the registrationroller 17, the black developing roller 31K, the heating roller 43, andthe reverse conveyance rollers 55) which are in the printer 1 and rotatein one direction, but also as a motor for generating rotational drivingforce to be transmitted to the switchback roller 50 and the intermediatesheet discharge roller 48.

Therefore, it is possible to prevent the number of motors in the printer1 from increasing, and while it is possible to reduce the cost andnoise, it is possible to switch the rotation direction of the switchbackroller 50 between the normal rotation direction and the reverse rotationdirection, thereby forming images on one side and the other side of asheet P.

(2) Further, according to the printer 1, as shown in FIGS. 5 and 10, thecam 147 presses the frame portion 116 of the holder 99 to swing theholder 99, so that the pendulum gear 100 rotatably supported on theholder 99 is moved.

Therefore, by pressing the holder 99 by the cam 147 such that thependulum gear 100 is moved, it is possible to switch the pendulum gear100 among the first engagement position, the second engagement positionand the disengagement position.

(3) Further, according to the printer 1, as shown in FIGS. 5 and 10, byengaging the toothed portion 134 with the drive gear 98 such that thepartially toothed gear 130 rotates with rotation of the drive gear 98,thereby moving the cam 147 to press the holder 99, it is possible tomove the pendulum gear 100. Also, as shown in FIGS. 7A, 7B, 12A, and12B, by causing the non-tooth portion 133 to face the drive gear 98 toprevent the partially toothed gear 130 from receiving the rotationaldriving force from the main motor 68, it is possible to stop therotation of the partially toothed gear 130 such that the holder 99 isnot pressed, thereby stopping movement of the pendulum gear 100.

Therefore, by engaging the toothed portion 134 with the drive gear 98such that the partially toothed gear 130 rotates with rotation of thedrive gear 98, it is possible to switch the pendulum gear 100 among thefirst engagement position, the second engagement position and thedisengagement position. Then, by stopping the partially toothed gear 130such that the non-tooth portion 133 faces the drive gear 98, it ispossible to hold the pendulum gear 100 at each engaging portion, therebyholding the normal rotation mode, the reverse rotation mode and thestack mode.

(4) Further, according to the printer 1, as shown in FIGS. 7A and 7B,the first non-tooth portion 135 corresponds to the normal rotation modewhose use frequency is relatively high, and as shown in FIGS. 12A, 12B,16A, and 16B, the second non-tooth portion 136 corresponds to thereverse rotation mode and the stack mode whose use frequencies arerelatively low, and therefore, t is possible to make the non-toothportion 133 correspond to each mode according to a use frequency.Therefore, it is possible to effectively suppress an increase in thesize of the partially toothed gear 130.

(5) Further, according to the printer 1, as shown in FIGS. 6 and 11,while being in engagement with the drive gear 98 so as to be able toalways transmit the one-direction rotational driving force, the pendulumgear 100 can move to the first engagement position of FIG. 4 where thependulum gear 100 is engaged with the first gear train, the secondengagement position of FIG. 9 where the pendulum gear 100 is engagedwith the second gear train, and the disengagement position of FIG. 13where the pendulum gear 100 is not engaged with any of the first geartrain and the second gear train.

That is, as shown in FIGS. 6 and 11, while always rotating in onedirection, the pendulum gear 100 can be switched among the firstengagement position, the second engagement position and thedisengagement position, thereby being capable of switching theswitchback roller 50 among rotation in the normal rotation direction,rotation in the reverse rotation direction, and a non-rotating state.

(6) Further, according to the printer 1, as shown in FIGS. 6 and 11,since the locking unit (the lever 103, the solenoid switch 104, and thecylindrical unit 131 of the sector gear 101) causes the non-toothportion 133 of the partially toothed gear 130 in the normal rotationmode, the reverse rotation mode and the stack mode to face the drivegear 98 against the biasing force of the torsion spring 148 biasing thepartially toothed gear 130, it is possible to prevent the driving forcefrom the main motor 68 from being transmitted to the partially toothedgear 130.

Therefore, it is possible to surely hold the normal rotation mode, thereverse rotation mode and the stack mode of the switching unit 83.

Meanwhile, in a case where facing of the non-tooth portion 133 and thedrive gear 98 by the locking unit is released, since it is possible tobias the partially toothed gear 130 by the biasing force of the torsionspring 148 in a direction in which the partially toothed gear 130 isrotated by the drive gear 98, it is possible to surely transmit therotational driving force from the main motor 68 to the partially toothedgear 130.

(7) Further, according to the printer 1, as shown in FIGS. 5 and 6,since the cam 147 has the I-shaped cam 146 for pressing the holder 99,and the V-shaped cam 145 which is biased by the torsion spring 148, itis possible to surely switch the mode of the switching unit 83.

(8) Further, according to the printer 1, as shown in FIGS. 8A to 8D,since the cam 147 and the partially toothed gear 130 are integrallyformed, it is possible to reduce the number of components.

(9) Further, according to the printer 1, as shown in FIGS. 7A, 7B, 12A,and 12B, engaging of the lever 103 with the protruding portion 140 ofthe cylindrical unit 131 and releasing of the lever 103 from theprotruding portion 140 are switched by the solenoid switch 104, androtation of the partially toothed gear 130 is regulated by engaging ofthe lever 103 and the protruding portion 140, and the partially toothedgear 130 is rotated by releasing engaging of the lever 103 and theprotruding portion 140.

Therefore, by switching of the solenoid switch 104, it is possible toswitch the partially toothed gear 130 between a rotation regulated stateand a rotating state.

(10) Further, according to the printer 1, as shown in FIGS. 7A, 7B, 12A,and 12B, if the lever 103 moves to the first engagement position and thesecond engagement position by switching of the solenoid switch 104,engaging of the first engagement portion 153 with the first protrudingportion 141 is released and the partially toothed gear 130 rotates.However, the second engagement portion 154 is engaged with the firstprotruding portion 141, whereby rotation of the partially toothed gear130 is regulated. That is, after engaging of the first engagementportion 153 with the first protruding portion 141 is released, thepartially toothed gear 130 rotates until the second engagement portion154 is engaged with the first protruding portion 141.

Also, as shown in FIGS. 12A, 12B, 16A, and 16B, if the lever 103 movesfrom the second engagement position to the first engagement position byswitching of the solenoid switch 104, engaging of the second engagementportion 154 with the first protruding portion 141 is released and thepartially toothed gear 130 rotates. However, the first engagementportion 153 is engaged with the second protruding portion 142, wherebyrotation of the partially toothed gear 130 is regulated. That is, afterengaging of the second engagement portion 154 with the first protrudingportion 141 is released, the partially toothed gear 130 rotates untilthe first engagement portion 153 is engaged with the second protrudingportion 142.

As described above, by switching the lever 103 between engaging with theprotruding portion 140 and releasing from the protruding portion 140 bythe solenoid switch 104, it is possible to repeat the rotation regulatedstate and rotating state of the partially toothed gear 130.

(11) Further, according to the printer 1, rotation of the partiallytoothed gear 130 is regulated at three positions, that is, a positionwhere the first protruding portion 141 and the first engagement portion153 are engaged with each other as shown in FIGS. 7A and 7B, a positionwhere the first protruding portion 141 and the second engagement portion154 are engaged with each other as shown in FIGS. 12A and 12B, and aposition where the second protruding portion 142 and the firstengagement portion 153 are engaged with each other as shown in FIGS. 16Aand 16B.

That is, since the three positions correspond to the normal rotationmode, the reverse rotation mode and the stack mode, respectively,switching to each mode becomes possible.

(12) Further, according to the printer 1, as shown in FIGS. 12A, 12B,16A, and 16B, since the second protruding portion 142 and the secondengagement portion 154 are disposed at positions where they are deviated(do not overlap) in the axial direction of the cylindrical unit 131formed at the sector gear 101, it is possible to surely prevent thesecond protruding portion 142 and the second engagement portion 154 frombeing engaged with each other.

(13) Further, according to the printer 1, as shown in FIGS. 10 and 14,by switching the lever 103 between the first engagement position and thesecond engagement position by the solenoid switch 104, it is possible toswitch the switching unit 83 from the normal rotation mode to thereverse rotation mode, and from the reverse rotation mode to the stackmode.

(14) Further, according to the printer 1, in a case of direct switchingfrom the stack mode to the reverse rotation mode, the pendulum gear 100moves from the disengagement position shown in FIG. 13 to the firstengagement position shown in FIG. 4, and moves from the first engagementposition to the second engagement position shown in FIG. 9, wherebyswitching from the stack mode to the reverse rotation mode is performed.

However, while direct switching from the stack mode to the reverserotation mode is performed, the pendulum gear 100 is not held at thefirst engagement position, and the switching unit 83 does not become thenormal rotation mode. Therefore, even though the pendulum gear 100passes the first engagement position, it is possible to surely performswitching from the stack mode to the reverse rotation mode.

(15) Further, according to the printer 1, as shown in FIGS. 8A to 8D,the partially toothed gear 130, the cylindrical unit 131, and the cam147 are integrally configured as the sector gear 101.

Therefore, it is possible to integrally configure various components forswitching among the normal rotation mode, the reverse rotation mode andthe stack mode, as one sector gear 101.

As a result, it is possible to simplify configurations while reducingthe number of components.

(16) Further, according to the printer 1, as shown in FIGS. 2A and 2B,it is possible to transmit the one-direction rotational driving force ofthe main motor 68 to each of the rotary bodies (the sheet feeding roller14, the conveying roller 16, the registration roller 17, the blackdeveloping roller 31K, the heating roller 43, and the reverse conveyancerollers 55) and each of the switchback roller 50 and the intermediatesheet discharge roller 48.

Further, while it is possible to always rotate each rotary body in onedirection by the one-direction rotational driving force of the mainmotor 68, it is possible to switch the rotation direction of each of theswitchback roller 50 and the intermediate sheet discharge roller 48between the normal rotation direction and the reverse rotationdirection.

6. Initial Control of Switching Unit by CPU

As shown in FIG. 17, the printer 1 includes the CPU 72 (an example of acontroller) for controlling the solenoid switch 104 such that thesolenoid switch 104 is switched between the excited state and thenon-excited state as described above.

The CPU 72 can perform first control to control the solenoid switch 104to hold the non-excited state for a first time period, second control tocontrol the solenoid switch 104 to hold the excited state for a secondtime period, and third control to control the solenoid switch 104 tohold the excited state for a third time period.

Here, the first time period is 0.12 sec or more, and is a time periodlonger than a longer time period between a time period while the sectorgear 101 rotates to a position where the first protruding portion 141comes into contact with the first engagement portion 153 after contactof the first engagement portion 153 and the second engagement surface144 is released and a time period while the sector gear 101 rotates to aposition where the second engagement surface 144 comes into contact withthe first engaging claw 158 after contact of the first engaging claw 158and the first engagement surface 143 is released.

The second time period is 0.13 sec or more, and is a time period longerthan a time period while the sector gear 101 rotates to a position wherethe first engagement surface 143 comes into contact with the secondengaging claw 159 after contact of the first engaging claw 158 and thesecond engagement surface 144 is released.

The third time period is 0.01 sec to 0.05 sec, and is a time periodwhich is longer than a time period while it is possible to surelyrelease contact of the first engaging claw 158 and the second engagementsurface 144 and which is shorter than a time period while the sectorgear 101 rotates to a position where the first engagement surface 143comes into contact with the first engaging claw 158 after contact of thefirst engaging claw 158 and the second engagement surface 144 isreleased. That is, the third time period is shorter than the second timeperiod.

The CPU 72 performs control to switch the solenoid switch 104 betweenthe excited state and the non-excited state for performing adouble-sided image forming process on a sheet P, separately from thefirst control, the second control, and the third control.

(1) Discharging of Sheet Remaining in Main Body Casing at Power-on

Immediately after power-on, in the printer 1, the solenoid switch 104 isalways controlled by the CPU 72 to become the non-excited state.

After the printer 1 is powered on, first, the main motor 68 is driven.

Therefore, the main motor 68 transmits the one-direction rotationaldriving force to the input gear 79 through the plurality of gears (notshown) of the main body casing 2.

Then, the one-direction rotational driving force having been transmittedto the input gear 79 is transmitted to the pendulum gear 100 through thedrive gear 98.

At this time, since the solenoid switch 104 is controlled to become thenon-excited state, the switching unit 83 becomes any one mode of thenormal rotation mode in which the pendulum gear 100 is held at the firstengagement position and the stack mode in which the pendulum gear 100 isheld at the disengagement position.

As shown in FIG. 18, after the printer 1 is powered on, the CPU 72performs the first control to hold the solenoid switch 104 in thenon-excited state for the first time period. The first time period inthe first control after the printer 1 is powered on is longer than atime period while a sheet P is discharged from a post-fixing sensor 63onto the sheet discharge tray 51. Incidentally, the first control ofthis illustrative embodiment may include control to issue an instructionfor the solenoid switch 104 to hold the non-excited state, or controlnot to issue an instruction for the solenoid switch 10 to the excitedstate.

Therefore, in a case where the switching unit 83 is in the normalrotation mode at power-on of the printer 1, the intermediate sheetdischarge roller 48 and the switchback roller 50 rotate in their normalrotation directions, such that even when a sheet P having not beendetected by the post-fixing sensor 63 and a sheet discharge sensor 64remains between the post-fixing sensor 63 and the sheet discharge sensor64 (to be described below) inside of the main body casing 2, the sheet Pis discharged. A case where a sheet P cannot be detected may include acase where the length of a sheet P is shorter than a distance betweenthe post-fixing sensor 63 and the sheet discharge sensor 64.

In a case where the switching unit 83 is in the stack mode at power-onof the printer 1, the intermediate sheet discharge roller 48 and theswitchback roller 50 do not rotate not only in their normal rotationdirections but also in their reverse rotation directions. Therefore,when there is a remaining sheet P which cannot be detected, the sheet Pis not conveyed to anywhere and continues to remain in the main bodycasing 2.

Subsequently, the CPU 72 performs the third control to control thesolenoid switch 104 to hold the excited state for the third time period.

As a result, engaging of the first engaging claw 158 of the lever 103with the protruding portion 140 is released, and the partially toothedgear 130 rotates. More specifically, in a case where the switching unit83 is in the normal rotation mode at power-on of the printer 1, as shownin FIGS. 7A and 7B, engaging of the first engaging claw 158 of the lever103 with the first engagement surface 143 of the first protrudingportion 141 is released, and the sector gear 101 rotates. Also, in acase where the switching unit 83 is in the stack mode at power-on of theprinter 1, as shown in FIGS. 16A and 16B, engaging of the first engagingclaw 158 of the lever 103 with the second engagement surface 144 of thesecond protruding portion 142 is released, and the sector gear 101rotates.

Subsequently, the CPU 72 performs the first control to control thesolenoid switch 104 to hold the excited state for the first time period,again.

Since the third time period of the third control is a short time from0.01 sec to 0.05 sec, as shown in FIGS. 7A, 7B, 16A, and 16B, as seen ina left side view, if the pendulum gear 100 rotates, immediately afterthe protruding portion 140 engaged with the first engaging claw 158passes under the first engaging claw 158, the first engaging claw 158 ispositioned at the first engagement position, again.

Therefore, in a case where the switching unit 83 is in the stack modeimmediately after power-on, the sector gear 101 rotates by about 210°such that the first engaging claw 158 is engaged with the firstengagement surface 143 of the first protruding portion 141, whereby theswitching unit 83 is switched to the normal rotation mode.

Then, the intermediate sheet discharge roller 48 and the switchbackroller 50 rotate in their normal rotation directions, and a sheet Phaving not been discharged in the stack mode is discharged.

Also, in a case where the switching unit 83 is in the normal rotationmode immediately after power-on, the sector gear 101 rotates by about150° such that the first engaging claw 158 is engaged with the secondengagement surface 144 of the second protruding portion 142, whereby theswitching unit 83 is switched to the stack mode. At this time, the sheetP has been already discharged.

Subsequently, the CPU 72 performs a start-up process of the printer 1.

(2) Mode Detection

As described above and shown in FIG. 18, at power-on of the printer 1,and/or after discharging of a sheet P remaining in the main body casing,the CPU 72 performs detection on the mode of the switching unit 83 todetermine whether the switching unit 83 is in the normal rotation modeor in the stack mode.

In order to perform mode detection, after discharging of a sheet Premaining in the main body casing 2 at power-on, the CPU 72 performs thesecond control to control the solenoid switch 104 to hold the excitedstate for the second time period.

Therefore, in a case where the switching unit 83 is in the normalrotation mode immediately before the second control is performed, theswitching unit 83 is switched to the reverse rotation mode.

Also, in a case where the switching unit 83 is in the stack modeimmediately before the second control is performed, as shown in FIGS.16A and 16B, the sector gear 101 rotates from a state where the secondprotruding portion 142 of the cylindrical unit 131 faces the lower frontside, specifically, a direction of about four o'clock as seen in a leftside view, by about 330° in a clockwise direction as seen in a left sideview, such that the first engagement surface 143 of the first protrudingportion 141 is engaged with the second engaging claw 159, whereby theswitching unit 83 is switched to the reverse rotation mode.

Incidentally, in a case where the switching unit 83 is switched from thestack mode to the reverse rotation mode, since the second protrudingportion 142 overlaps the second engaging claw 159 of the lever 103 asseen in a left side view in the middle of rotation of the sector gear101, and the second protruding portion 142 and the second engaging claw159 are deviated from each other in the left-right direction so as notto overlap as seen from a direction perpendicular to the rotation axisdirection of the cylindrical unit 131, the second protruding portion 142and the second engaging claw 159 are not engaged with each other, andthe sector gear 101 receives the rotational driving force of the drivegear 98, thereby rotating. Also, as seen in a left side view in themiddle of rotation of the sector gear 101, the first protruding portion141 passes under the first engaging claw 158. At this time, as shown inFIG. 5, as seen in a left side view, the I-shaped cam 146 of the sectorgear 101 is directed to about three o'clock with respect to the sectorgear shaft 125. Therefore, the holder 99 is biased in a counterclockwisedirection as seen in a left side view by the biasing force of thetension spring 121, whereby the pendulum gear 100 is positioned at thefirst engagement position, and the intermediate sheet discharge roller48 and the switchback roller 50 are simultaneously rotated in theirnormal rotation directions.

Therefore, after the switching unit 83 is switched to the reverserotation mode, the solenoid switch 104 is switched to the non-excitedstate by the CPU 72, whereby the switching unit 83 is switched to thestack mode.

As a result, detection on the mode of the switching unit 83 by the CPU72 is completed.

7. Effects of Control of CPU on Switching Unit

(1) According to the printer 1, as shown in FIGS. 4 and 9, since it isunnecessary to switch the rotation direction of the rotational drivingforce of the main motor 68 for switching the rotation directions of theswitchback roller 50 and the intermediate sheet discharge roller 48, itis possible to use the main motor 68 not only as a motor for generatingrotational driving force for rotating the rotary bodies (the sheetfeeding roller 14, the conveying roller 16, the registration roller 17,the black developing roller 31K, the heating roller 43, and the reverseconveyance rollers 55) which are in the printer 1 and rotate in onedirection, but also as a motor for generating rotational driving forceto be transmitted to the switchback roller 50.

Meanwhile, according to the printer 1, the solenoid switch 104 can beselectively switched between the non-excited state allowing switching ofthe switching unit 83 to the normal rotation mode or the stack mode, andthe excited state allowing switching of the switching unit 83 to thereverse rotation mode. The CPU 72 controls the switching of the solenoidswitch 104 between the non-excited state and the excited state.

Therefore, there may be problems in which the CPU 72 cannot determinewhether the switching unit 83 is in the normal rotation mode or in thestack mode, only by switching the solenoid switch 104 to the non-excitedstate, and before switching the switching unit 83 to the reverserotation mode such that the switchback roller 50 is rotated in thereverse rotation direction, the CPU 72 cannot switch the switching unit83 to the normal rotation mode such that the switchback roller 50 isrotated in the normal rotation direction, whereby a sheet P isdischarged to the outside of the printer 1.

Accordingly, in the printer 1, as shown in FIG. 18, the CPU 72 canperform the first control to control the solenoid switch 104 to hold thenon-excited state for the first time period, the second control tocontrol the solenoid switch 104 to hold the excited state for the secondtime period, and the third control to control the solenoid switch 104 tohold the excited state for the third time period shorter than the secondtime.

As a result, by performing the third control on the switching unit 83having been switched to the normal rotation mode or the stack mode bythe first control of the CPU 72, it is possible to interchange thenormal mode and the stack mode.

Accordingly, if the first control and the third control are performedbefore the second control is performed, it is possible to necessarilyperform the normal rotation mode before performance of the reverserotation mode.

Therefore, while it is possible to use the main motor 68 not only as amotor for generating the one-direction rotational driving force forrotating the switchback roller 50 and the intermediate sheet dischargeroller 48 but also as a motor for generating rotational driving forcefor rotating the rotary bodies which are in the printer 1 and rotate inone direction, thereby reducing the cost and noise, it is possible tosurely switch the switching unit 83 to the normal rotation mode beforethe reverse rotation mode such that the switchback roller 50 and theintermediate sheet discharge roller 48 are rotated in their normalrotation directions, whereby a sheet P is discharged.

(2) Further, according to the printer 1, as shown in FIG. 18,immediately after power-on of the printer 1, it is possible to switchthe switching unit 83 to the normal rotation mode such that theswitchback roller 50 is rotated in the normal rotation direction,whereby a sheet P is discharged.

Therefore, immediately after power-on of the printer 1, even when thereis a remaining sheet P in the printer 1, it is possible to forcedlydischarge the sheet P.

(3) Further, according to the printer 1, as shown in FIG. 1, in a casewhere it is possible to detect whether there is a remaining sheet P bythe post-fixing sensor 63 in the middle of conveyance path from theimage forming unit 4 to the discharge opening 49, an appropriate processfor discharging the sheet P is performed, it is possible to perform thesecond control, thereby switching the switching unit 83 to the reverserotation mode, and then perform a double-sided image forming process.

Meanwhile, there may be a problem in which when there is a remainingsheet P on the downstream side from the post-fixing sensor 63 in theconveyance direction in the middle of conveyance path from the imageforming unit 4 to the discharge opening 49 of the primary conveyancepath 52, it is not possible to detect existence or non-existence of thesheet P by the post-fixing sensor 63.

However, in the printer 1, regardless of detection of the sheet P by thepost-fixing sensor 63, before the second control, it is possible toperform the normal rotation mode for the first time period longer thanthe conveyance time of the sheet P while the sheet P is conveyed fromthe post-fixing sensor 63 to the discharge opening 49.

Therefore, before switching the switching unit 83 to the reverserotation mode, it is possible to surely discharge the sheet P.

(4) Also, according to the printer 1, as shown in FIGS. 4 and 9, sincethe solenoid switch 104 is used as the switching element, it is possibleto selectively switch the switching unit 83 between the non-excitedstate and the excited state by a simple configuration.

Therefore, a switching element having a complicated configuration is notnecessary, and thus, it is possible to reduce the cost.

(5) Further, according to the printer 1, as shown in FIG. 1, in theprinter 1, in a case of forming an image only on one side of a sheet P,since it is not necessary to switch the switchback roller 50 to thereverse rotation direction, it is possible to form the image on thesheet P only in the normal rotation mode without switching the switchingunit 83 to the reverse rotation mode.

Further, according to the printer 1, since the normal rotation modecorresponds to the non-excited state of the solenoid switch 104, it ispossible to suppress consumption of electric power which is applied tothe solenoid switch 104 in a case of forming an image only on one sideof the sheet P.

(6) Further, according to the printer 1, as shown in FIG. 17, it ispossible to control the CPU 72 such that the solenoid switch 104 becomesthe non-excited state, whereby the switching unit 83 is switched to thenormal rotation mode, and it is possible to control the CPU 72 such thatthe solenoid switch 104 becomes the excited state, whereby the switchingunit 83 is switched from the normal rotation mode to the reverserotation mode, and it is possible to control the CPU 72 such that thesolenoid switch 104 becomes the non-excited state, whereby the switchingunit 83 is switched from the reverse rotation mode to the stack mode.

Accordingly, by a simple operation of controlling the CPU 72 such thatthe solenoid switch 104 becomes the non-excited state or the excitedstate, it is possible to perform switching among the normal rotationmode, the reverse rotation mode and the stack mode.

(7) According to the printer 1, as shown in FIGS. 4 and 9, since it isnot necessary to switch the rotation direction of the rotational drivingforce of the main motor 68 for switching the rotation direction of theswitchback roller 50, it is possible to use the main motor 68 not onlyas a motor for generating rotational driving force for rotating therotary bodies (the sheet feeding roller 14, the conveying roller 16, theregistration roller 17, the black developing roller 31K, the heatingroller 43, and the reverse conveyance rollers 55) which are in theprinter 1 and rotate in one direction, but also as a motor forgenerating rotational driving force to be transmitted to the switchbackroller 50.

The CPU 72 performs control such that the solenoid switch 104 isselectively switched between the non-excited state allowing switching ofthe switching unit 83 to the normal rotation mode or the stack mode, andthe excited state allowing switching of the switching unit 83 to thereverse rotation mode.

Further, as shown in FIG. 18, since the switching unit 83 can beswitched from the reverse rotation mode only to the stack mode, in acase where the control unit 70 controls the switching unit 83, therebyperforming an image forming operation, first, the switching unit 83 isswitched to the reverse rotation mode. Then, if the switching unit 83 isswitched from the reverse rotation mode to the stack mode, it ispossible to set an initial mode using the timing of the switching as thereference of control.

As a result, while it is possible to reduce the cost and noise, it ispossible to perform the image forming operation using switching of theswitching unit 83 from the reverse rotation mode to the stack mode asthe reference of control.

(8) Further, according to the printer 1, as shown in FIG. 18, after theprinter 1 is powered up and before an image is formed on a sheet P, itis possible to switch the switching unit 83 from the reverse rotationmode to the stack mode, and set the reference of control.

8. Double-Sided Image Forming Process

A double-sided image forming process of the CPU 72 on a plurality ofsheets P will be described with reference to FIG. 19.

As shown in FIG. 1, the main body casing 2 includes, in the primaryconveyance path 52, a sheet feeding sensor 60, a pre-registration sensor61, a post-registration sensor 62, the post-fixing sensor 63 and thesheet discharge sensor 64, and further includes a reverse-path sensor 65in the secondary conveyance path 56.

The sheet feeding sensor 60 is positioned in the vicinity of the sheetfeeding roller 14 in the main body casing 2.

The pre-registration sensor 61 is positioned on the downstream side fromthe conveying roller 16 in the conveyance direction of the sheets P andon the upstream side from the registration roller 17 in the conveyancedirection of the sheets P, in the primary conveyance path 52 of the mainbody casing 2.

The post-registration sensor 62 is positioned on the downstream sidefrom the registration roller 17 in the conveyance direction of thesheets P and on the upstream side from a section between the foremostphotosensitive drum 28 and the conveyor belt 39 in the conveyancedirection of the sheets P, in the primary conveyance path 52 of the mainbody casing 2.

The post-fixing sensor 63 is positioned on the downstream side from thefixing unit 23 in the conveyance direction of the sheets P and on theupstream side from the intermediate sheet discharge roller 48 in theconveyance direction of the sheets P, in the primary conveyance path 52of the main body casing 2.

The sheet discharge sensor 64 is positioned in the vicinity of theswitchback roller 50 on the upstream side from the switchback roller 50in the conveyance direction of the sheets P, in the primary conveyancepath 52 of the main body casing 2.

The reverse-path sensor 65 is positioned in the vicinity of the rearmostreverse conveyance roller 55 in the main body casing 2.

Further, each of the sheet feeding sensor 60, the pre-registrationsensor 61, the post-registration sensor 62, the post-fixing sensor 63,the sheet discharge sensor 64, and the reverse-path sensor 65 isconfigured to have an actuator capable of swinging such that theactuator is inclined and turned on by contact with a sheet P, and isturned off by separation from a sheet P. Further, each sensor isconfigured to transmit a detection signal of ON/OFF of a correspondingactuator to the CPU 72.

The double-sided image forming process of the CPU 72 on the plurality ofsheets P is performed with a set of two sheets.

Of two sheets P of one set, a sheet P on which an image is formed firstis referred to as a preceding sheet P1 (an example of a first recordingmedium), and a sheet P on which an image is formed second is referred toas a succeeding sheet P2 (an example of a second recording medium).

In the each of the preceding sheet P1 and the succeeding sheet P2, aside on which an image is formed first is earlier to as one side, and aside on which an image is formed later is referred to as the other side.

The CPU 72 performs a first step of holding the normal rotation modesuch that the preceding sheet P1 is fed from the sheet feeding unit 3 tothe primary conveyance path 52 by the conveying roller 16, an image isformed on one side of the preceding sheet P1 by the image forming unit4, and the preceding sheet P1 is conveyed to the switchback roller 50.

Specifically, before performing the first step, mode detection iscompleted, the start-up process is completed, and then the process motor69 is driven.

Subsequently, the CPU 72 performs control so as to hold the switchingunit 83 in the normal rotation mode.

Then, as shown in FIG. 20A, the preceding sheet P1 on the sheet feedingtray 12 of the sheet feeding unit 3 is conveyed toward between thephotosensitive drums 28 and the conveyor belt 39 as described above.

At this time, the succeeding sheet P2 is stacked on the sheet feedingtray 12 of the sheet feeding unit 3.

Therefore, as shown at a timing A in FIG. 19, the sheet feeding sensor60, the pre-registration sensor 61, and the post-registration sensor 62are turned on.

Next, while the preceding sheet P1 is conveyed in the primary conveyancepath 52, as shown in FIG. 20B, an image is formed on one side of thepreceding sheet P1 by the image forming unit 4 as described above. Thepreceding sheet P1 passes through the fixing unit 23 and is conveyed bythe intermediate sheet discharge roller 48 and the switchback roller 50such that the leading end of the preceding sheet P1 (an end portion onthe upstream side in the conveyance direction in the primary conveyancepath 52) is positioned in the vicinity of the discharge opening 49.

At this time, the succeeding sheet P2 is stacked on the sheet feedingtray 12 of the sheet feeding unit 3.

As a result, as shown at a timing B in FIG. 19, the sheet feeding sensor60, the pre-registration sensor 61, and the post-registration sensor 62are turned off, and the post-fixing sensor 63 and the sheet dischargesensor 64 are turned on.

Next, as shown in FIG. 20C, the preceding sheet P1 is conveyed to aposition where the trailing end of the preceding sheet (an end portionon the downstream side in the conveyance direction in the primaryconveyance path 52) is in the vicinity of the discharge opening 49.

At this time, the succeeding sheet P2 is stacked on the sheet feedingtray 12 of the sheet feeding unit 3.

Therefore, as shown at a timing C in FIG. 19, the sheet discharge sensor64 is maintained in the ON state, and the post-fixing sensor 63 isturned off.

Then, if a predetermined time period elapses from turning on of thesheet discharge sensor 64 due to the preceding sheet P1, the CPU 72performs a second step of holding the reverse rotation mode such thatthe preceding sheet P1 is conveyed into the secondary conveyance path56.

Specifically, in order to perform the second step, at a timing when 1.00sec elapses from turning on of the sheet discharge sensor 64, the CPU 72performs control such that the switching unit 83 is switched from thenormal rotation mode to the reverse rotation mode.

As a result, the preceding sheet P1 is reversed and is conveyed towardthe secondary conveyance path 56.

Then, as shown in FIG. 20D, the preceding sheet P1 is conveyed such thatthe leading end of the preceding sheet (an end portion on the downstreamside in the conveyance direction in the secondary conveyance path 56) ispositioned in the vicinity of the rearmost reverse conveyance roller 55.

After the switching unit 83 is switched from the normal rotation mode tothe reverse rotation mode, when a predetermined time period elapses, theCPU 72 drives the sheet feeding roller 14. Therefore, after thepredetermined time period elapses, the succeeding sheet P2 is conveyedtoward between the photosensitive drums 28 and the conveyor belt 39.

Then, as shown at a timing D in FIG. 19, the sheet discharge sensor 64is turned off, and the sheet feeding sensor 60, the pre-registrationsensor 61, and the post-registration sensor 62 are turned on.

The CPU 72 performs a third step of holding the stack mode so as to keepthe preceding sheet P1 in the secondary conveyance path 56 such that thepreceding sheet P1 which is conveyed in the secondary conveyance path 56does not catch up with the succeeding sheet P2 in the middle of thesecond step.

Specifically, although the switching unit 83 has been switched to thereverse rotation mode by the second step, after the succeeding sheet P2passes the post-registration sensor 62, the switching unit 83 isswitched to the stack mode such that the preceding sheet P1 is kept inthe secondary conveyance path 56, until a predetermined time periodelapses. After the post-registration sensor 62 is turned on due to thesucceeding sheet P2, if a predetermined time elapses, the CPU 72switches the switching unit 83 from the stack mode to the reverserotation mode.

Further, after the post-registration sensor 62 is turned on due to thesucceeding sheet P2, when a predetermined time period elapses, the CPU72 performs a fourth step of holding the normal rotation mode such thatthe succeeding sheet P2 is conveyed to the switchback roller 50.

Specifically, in performing the fourth step, since the preceding sheetP1 has been conveyed toward the secondary conveyance path 56, and when apredetermined time period has elapsed from turning on of thereverse-path sensor 65, the entire preceding sheet P1 has entered thesecondary conveyance path 56, and has passed the intermediate sheetdischarge roller 48, the CPU 72 performs control such that the switchingunit 83 is switched from the reverse rotation mode to the normalrotation mode through the stack mode.

Accordingly, an image is formed on one side of the succeeding sheet P2,which is conveyed toward the discharge opening 49 by the intermediatesheet discharge roller 48 rotating in the normal rotation direction.

Then, as shown in FIG. 21E, the preceding sheet P1 is conveyed to aposition where the trailing end of the preceding sheet (an end portionon the upstream side in the conveyance direction in the secondaryconveyance path 56) exceeds the rearmost reverse conveyance roller 55.

The succeeding sheet P2 is conveyed to a position by the intermediatesheet discharge roller 48 and the switchback roller 50 48 rotating intheir normal rotation directions such that the leading end of thesucceeding sheet (an end portion on the downstream side in theconveyance direction in the primary conveyance path 52) is positioned inthe vicinity of the discharge opening 49.

Then, as shown at a timing E in FIG. 19, the reverse-path sensor 65 isturned off, and the post-fixing sensor 63 and the sheet discharge sensor64 are turned on.

Next, as shown in FIG. 21F, the preceding sheet P1 is conveyed such thatthe leading end of the preceding sheet (an end portion on the downstreamside in the conveyance direction in the secondary conveyance path 56) ispositioned in the vicinity of the conveying roller 16.

The succeeding sheet P2 is conveyed such that the trailing end of thesucceeding sheet (an end portion on the downstream side in theconveyance direction in the primary conveyance path 52) is positioned inthe vicinity of the discharge opening 49.

At this time, as shown at a timing F in FIG. 19, the sheet dischargesensor 64 is maintained in the ON state, and the post-fixing sensor 63is turned off.

Next, the CPU 72 performs a fifth step of holding the reverse rotationmode such that the succeeding sheet P2 is conveyed into the secondaryconveyance path 56.

Specifically, in performing the fifth step, when 1.00 sec elapses fromturning on of the sheet discharge sensor 64, the CPU 72 performs controlsuch that the switching unit 83 is switched from the normal rotationmode to the reverse rotation mode.

Therefore, the succeeding sheet P2 is reversed, and is conveyed towardthe secondary conveyance path 56.

Then, as shown in FIG. 21G, the succeeding sheet P2 is conveyed suchthat the leading end of the succeeding sheet (an end portion on thedownstream side in the conveyance direction in the secondary conveyancepath 56) is positioned in the vicinity of the rearmost reverseconveyance roller 55.

The preceding sheet P1 is conveyed into the primary conveyance path 52again by rotation of the conveying roller 16, and is conveyed towardbetween the photosensitive drums 28 and the conveyor belt 39.

Therefore, as shown at a timing G in FIG. 19, the sheet discharge sensor64 is turned off, and the reverse-path sensor 65 and thepost-registration sensor 62 are turned on.

Then, when a predetermined time period elapses from the turning on ofthe reverse-path sensor 65 due to the succeeding sheet P2, the CPU 72performs a sixth step of holding the normal rotation mode. The precedingsheet P1 in the primary conveyance path 52 is discharged from the mainbody casing 2 through the discharge opening 49, and the succeeding sheetP2 is conveyed from the secondary conveyance path 56 into the primaryconveyance path 52 by the conveying roller 16, an image is formed on theother side of the succeeding sheet P2 by the image forming unit 4, andthe succeeding sheet P2 is discharged from the main body casing 2through the discharge opening 49.

Specifically, in performing the sixth step, since the succeeding sheetP2 has been conveyed toward the secondary conveyance path 56, and when apredetermined time period has elapsed from turning on of thereverse-path sensor 65, the entire succeeding sheet P2 has entered thesecondary conveyance path 56, and has passed the intermediate sheetdischarge roller 48, the CPU 72 performs control such that the switchingunit 83 is switched from the reverse rotation mode to the normalrotation mode through the stack mode.

Therefore, an image is formed on the other side of the preceding sheetP1 having been conveyed to the sheet discharge unit 5, and the precedingsheet P1 is conveyed toward the discharge opening 49 by the intermediatesheet discharge roller 48 rotating in the normal rotation direction.

Then, as shown in FIG. 21H, the preceding sheet P1 is conveyed by theintermediate sheet discharge roller 48 and the switchback roller 50rotating in their normal rotation directions such that the leading endof the preceding sheet P1 (an end portion on the downstream side in theconveyance direction in the primary conveyance path 52) is positioned inthe vicinity of the discharge opening 49.

The succeeding sheet P2 is conveyed to a position where the trailing endof the succeeding sheet (on the upstream side in the conveyancedirection in the secondary conveyance path 56) exceeds the rearmostreverse conveyance roller 55.

Then, as shown at a timing H in FIG. 19, the reverse-path sensor 65 isturned off, and the post-fixing sensor 63 and the sheet discharge sensor64 are turned on.

Next, as shown in FIG. 221, the preceding sheet P1 is conveyed such thatthe trailing end of the preceding sheet (an end portion on thedownstream side in the conveyance direction in the primary conveyancepath 52) is positioned in the vicinity of the discharge opening 49.

The succeeding sheet P2 is conveyed such that the leading end of thesucceeding sheet (an end portion on the downstream side in theconveyance direction in the secondary conveyance path 56) is positionedin the vicinity of the conveying roller 16.

At this time, as shown at a timing I in FIG. 19, the sheet dischargesensor 64 is maintained in the ON state, and the post-fixing sensor 63is turned off.

Next, as shown in FIG. 22J, the preceding sheet P1 is discharged fromthe discharge opening 49 onto the sheet discharge tray 51.

The succeeding sheet P2 is conveyed into the primary conveyance path 52again by rotation of the conveying roller 16, and is conveyed towardbetween the rearmost photosensitive drum 28 and the conveyor belt 39.

At this time, as shown at a timing J in FIG. 19, the sheet dischargesensor 64 is turned off, and the pre-registration sensor 61 and thepost-registration sensor 62 are turned on.

Further, as shown in FIG. 22K, the preceding sheet P1 is loaded on thesheet discharge tray 51.

Next, the succeeding sheet P2 is conveyed by the intermediate sheetdischarge roller 48 and the switchback roller 50 rotating in theirnormal rotation directions such that the leading end of the succeedingsheet (on the downstream side in the conveyance direction in the primaryconveyance path 52) is positioned in the vicinity of the dischargeopening 49.

Therefore, as shown at a timing K in FIG. 19, the post-fixing sensor 63and the sheet discharge sensor 64 are turned on.

At this time, as shown in FIG. 22L, the preceding sheet P1 is loaded onthe sheet discharge tray 51.

Next, the succeeding sheet P2 is discharged from the discharge opening49 onto the sheet discharge tray 51, so as to be loaded on the precedingsheet P1.

Therefore, as shown at a timing L in FIG. 19, the post-fixing sensor 63and the sheet discharge sensor 64 are turned off.

As described above, the double-sided image forming process on two sheetsP of the first set is completed.

In a case of subsequently performing the double-sided image formingprocess on the second and subsequent sets, at the timing K of FIG. 19,as shown in FIG. 20A, a preceding sheet P1 of the second set is conveyedfrom the sheet feeding tray 12 toward between the photosensitive drums28 and the conveyor belt 39.

Then, when the preceding sheet P1 and the succeeding sheet P2 of thefirst set are loaded on the sheet discharge tray 51 as shown in FIG.22L, as shown in FIG. 20B, an image is formed on one side of thepreceding sheet P1 of the second set by the image forming unit 4. Thepreceding sheet P1 passes through the fixing unit 23 and is conveyed bythe intermediate sheet discharge roller 48 and the switchback roller 50such that the leading end of the preceding sheet (an end portion on theupstream side in the conveyance direction in the primary conveyance path52) is positioned in the vicinity of the discharge opening 49.

Thereafter, on the sheets P, images are formed by the similar process tothe double-sided image forming process on the preceding sheet P1 andsucceeding sheet P2 of the first set.

In a case where the number of the plurality of sheets P is odd, after animage is formed on one side of the final one sheet P, at a timing whenthe trailing end of the sheet P (an end portion on the upstream side inthe conveyance direction in the primary conveyance path 52) reaches thesheet discharge sensor 64, the switching unit 83 is switched from thenormal rotation mode to the reverse rotation mode, and the sheet P isconveyed toward the secondary conveyance path 56.

Thereafter, the leading end of the sheet P (an end portion on theupstream side in the conveyance direction in the secondary conveyancepath 56) reaches the rearmost reverse conveyance roller 55, whereby thereverse-path sensor 65 is turned on. Thereafter, when a predeterminedtime period elapses, the switching unit 83 is switched to the stackmode.

Then, while the sheet P is conveyed into the primary conveyance path 52again, an image is formed on the other side of the sheet P and the sheetP reaches the fixing unit 23, the switching unit 83 is switched from thestack mode to the normal rotation mode.

Thereafter, the sheet P is discharged onto the sheet discharge tray 51through the discharge opening 49 by the intermediate sheet dischargeroller 48 and the switchback roller 50 rotating in their normal rotationdirections.

As a result, the double-sided image forming process on both sides ofeach of the plurality of sheets P is completed.

9. Effects of Double-Sided Image Forming Process

According to the printer 1, as shown in FIGS. 20A to 20D, FIGS. 21E to21H and 22I to 22L, image forming on one side and the other side of eachof a preceding sheet P1 and a succeeding sheet P2 is performed in orderof one side of the preceding sheet P1, one side of the succeeding sheetP2, the other side of the preceding sheet P1, and the other side of thesucceeding sheet P2.

Therefore, as compared to a process of forming images on one side andthe other side of the preceding sheet P1 and then forming images on oneside and the other side of the succeeding sheet P2, it is possible toform images on the preceding sheet P1 and the succeeding sheet P2 in ashorter time.

As a result, while it is possible to reduce the cost and noise, it ispossible to efficiently form images on one side and the other side ofeach of the plurality of sheets P.

10. Other Illustrative Embodiments

As an example of the switchback roller, the switchback roller 50 forconveying a sheet P toward the sheet discharge tray 51 has beendescribed. However, the present invention is not limited thereto. Theintermediate sheet discharge roller 48 for switching the conveyancedirection of a sheet P in the main body casing 2 may be taken as anexample of the switchback roller.

In that case, a gear arrangement of the fourth intermediate gear 94 andthe intermediate sheet discharge roller gear 87 for transmittingrotational driving force for rotating the intermediate sheet dischargeroller 48 in the normal rotation direction may be taken as an example ofthe first gear train, and a gear arrangement of the fifth intermediategear 95, the second intermediate gear 92, the third intermediate gear93, the fourth intermediate gear 94, and the intermediate sheetdischarge roller gear 87 may be taken as an example of the second geartrain.

What is claimed is:
 1. An image forming apparatus comprising: a drivingsource configured to generate one-direction rotational driving force; aswitchback roller configured to be switched between a normal rotationdirection and a reverse rotation direction for switching a conveyancedirection of a recording medium having an image formed thereon by animage forming unit; a first gear train configured to transmit theone-direction rotational driving force of the driving source to theswitchback roller such that a rotation direction of the switchbackroller becomes the normal rotation direction; a second gear trainconfigured to transmit the one-direction rotational driving force of thedriving source to the switchback roller such that the rotation directionof the switchback roller becomes the reverse rotation direction; and aswitching unit including a pendulum gear configured to engage a geartrain for transmitting the one-direction rotational driving force of thedriving source with any of the first gear train and the second geartrain; wherein the pendulum gear is configured to be movable among: afirst engagement position where the pendulum gear is engaged with thefirst gear train, a second engagement position where the pendulum gearis engaged with the second gear train, and a disengagement positionwhere the pendulum gear is not engaged with any of the first gear trainand the second gear train, wherein the switching unit is configured tobe switchable among: a first mode in which the pendulum gear is held atthe first engagement position such that the one-direction rotationaldriving force of the driving source is transmitted to the first geartrain, a second mode in which the pendulum gear is held at the secondengagement position such that the one-direction rotational driving forceof the driving source is transmitted to the second gear train, and athird mode in which the pendulum gear is held at the disengagementposition such that the one-direction rotational driving force of thedriving source is not transmitted to any of the first gear train and thesecond gear train.
 2. The image forming apparatus according to claim 1,wherein the switching unit further includes: a holder configured tosupport the pendulum gear to be rotatable, and a cam configured to pressthe holder to move the holder, thereby moving the pendulum gearsupported on the holder.
 3. The image forming apparatus according toclaim 2, wherein the switching unit further includes: a drive gearconfigured to receive the one-direction rotational driving force fromthe driving source; and a partially toothed gear configured to be ableto receive the rotational driving force through the drive gear andoperate the cam, and wherein the partially toothed gear includes: atoothed portion configured to be engaged with the drive gear; and anon-tooth portion configured to face the drive gear in each of the firstmode, the second mode and the third mode.
 4. The image forming apparatusaccording to claim 3, wherein the non-tooth portion includes: a firstnon-tooth portion configured to face the drive gear in the first mode;and a second non-tooth portion configured to face the drive gear in thesecond mode and the third mode.
 5. The image forming apparatus accordingto claim 4, wherein the drive gear is configured to be engaged with thependulum gear, and wherein the holder is configured to swing around arotation center of the drive gear.
 6. The image forming apparatusaccording to claim 4, wherein the switching unit further includes: abiasing member configured to apply biasing force to the partiallytoothed gear in a direction in which the partially toothed gear isrotated by the drive gear; and a locking unit configured to hold a statewhere the non-tooth portion and the drive gear face each other in eachof the first mode, the second mode and the third mode.
 7. The imageforming apparatus according to claim 6, wherein the cam includes a firstcam and a second cam, and wherein the second cam is configured to bebiased by the biasing member when the first cam presses the holder. 8.The image forming apparatus according to claim 6, wherein the cam isformed integrally with the partially toothed gear.
 9. The image formingapparatus according to claim 6, wherein the locking unit includes: aregulating member configured to be rotatable in conjunction with thepartially toothed gear, and including a protruding portion formed at anouter periphery thereof; an engaging member configured to be engagedwith the protruding portion to regulate rotation of the partiallytoothed gear; and a switching element configured to switch betweenengaging and releasing of the engaging member with respect to theprotruding portion.
 10. The image forming apparatus according to claim9, wherein the engaging member includes a first engagement portion and asecond engagement portion and is configured to be movable between: afirst engagement position where the first engagement portion is engagedwith the protruding portion and the second engagement portion is notengaged with the protruding portion; and a second engagement positionwhere the second engagement portion is engaged with the protrudingportion and the first engagement portion is not engaged with theprotruding portion.
 11. The image forming apparatus according to claim10, wherein the protruding portion includes a first protruding portionand a second protruding portion, wherein the first protruding portion isconfigured to be able to engage with the first engagement portion whenthe engaging member is positioned at the first engagement position andto be able to engage with the second engagement portion when theengaging member is positioned at the second engagement position, andwherein the second protruding portion is configured to be able to engagewith the first engagement portion when the engaging member is positionedat the first engagement position and to be unable to engage with thesecond engagement portion when the engaging member is positioned at thesecond engagement position.
 12. The image forming apparatus according toclaim 11, wherein the second protruding portion and the secondengagement portion do not overlap with each other as seen in a directionperpendicular to a rotation axis of the regulating member.
 13. The imageforming apparatus according to claim 11, wherein the engaging member isconfigured to be movable by the switching element between a firstengagement position where the first engagement portion is engaged withthe protruding portion and a second engagement position where the secondengagement portion is engaged with the protruding portion, and whereinthe engaging member is positioned at the first engagement position andthe first engagement portion is engaged with the first protrudingportion, whereby the switching mechanism is switched to the first mode,wherein the engaging member is positioned at the second engagementposition and the second engagement portion is engaged with the firstprotruding portion, whereby the switching mechanism is switched from thefirst mode to the second mode, and wherein the engaging member ispositioned at the first engagement position and the first engagementportion is engaged with the second protruding portion, whereby theswitching mechanism is switched from the second mode to the third mode.14. The image forming apparatus according to claim 13, wherein when theswitching unit switches from the third mode to the second mode, thependulum gear is configured to pass the first engagement position wherethe pendulum gear is engaged with the first gear train without the firstengagement portion engaging with the first protruding portion.
 15. Theimage forming apparatus according to claim 9, wherein the switching unitincludes a switching gear which is integrally formed such that thepartially toothed gear, the regulating member and the cam rotate on asame axis.
 16. The image forming apparatus according to claim 1, furthercomprising: a conveying roller configured to convey a recording mediumhaving an image not formed yet toward the image forming unit, andwherein the driving source is configured to transmit the one-directionrotational driving force to the conveying roller.
 17. The image formingapparatus according to claim 1, further comprising: the image formingunit disposed on an upstream side of the switchback roller in theconveyance direction; a discharge opening, through which a recordingmedium having an image formed thereon is discharged; a conveying rollerdisposed on an upstream side of the image forming unit in the conveyancedirection, and configured to receive the rotational driving force fromthe driving source; a sheet feeding unit configured to load thereon arecording medium; a primary conveyance path along which a recordingmedium is conveyed by the conveying roller and reaches the switchbackroller through the image forming unit; a secondary conveyance path alongwhich a recording medium is conveyed from the switchback roller and jointhe primary conveyance path at an upstream side of the image formingunit in the conveyance direction; a switching element configured toselectively change between a first state allowing switching of theswitching unit into the first mode or the third mode, and a second stateallowing switching of the switching unit into the second mode; and acontroller configured to control a state of the switching element,wherein the controller is configured to perform a double-sided imageforming process for forming images on recording media, wherein thedouble-sided image forming process is a process of forming images onboth sides of each of a first recording medium and a second recordingmedium, and wherein the double-sided image forming process includes: afirst step of holding the first mode such that the first recordingmedium is fed from the sheet feeding unit into the primary conveyancepath by the conveying roller, an image is formed on one side of thefirst recording medium by the image forming unit, and the firstrecording medium is conveyed to the switchback roller; a second step ofholding the second mode such that the first recording medium is conveyedinto the secondary conveyance path; a third step of holding the thirdmode to stop conveyance of the first recording medium in the secondaryconveyance path; a fourth step of holding the first mode such that thesecond recording medium is conveyed to the switchback roller; a fifthstep of holding the second mode such that the second recording medium isconveyed into the secondary conveyance path; and a sixth step of holdingthe first mode such that the first recording medium in the primaryconveyance path is discharged from a discharge opening, the secondrecording medium is conveyed from the secondary conveyance path into theprimary conveyance path by the conveying roller, an image is formed onthe other side of the second recording medium, and the second recordingmedium is discharged from the discharge opening.